Congratulations on the start of your CP(carnivorous plant) obsession. These are great and hardy plants. I've got a mess of them and plan to make a bog garden soon.
I'll do my best to answer your questions. For your first question, those are three separate plants. It's also not a bad thing for them to be crowded. I'd typically only split them if the pot was super crowded of if I wanted to give some away. Here is a good example of a cool planting with tons of plants:
As for repotting (and care in general) I'd really recommend you pick up a copy of 'The Savage Garden' by Peter D'Amato
It's a great reference and will give you soil recipes for each plant and care instructions. I'll be up front with you and tell you I killed my first two VFT's(venus flytraps) and I live in SC, USA where they're native to. I found all kinds of trash info on the internet and this book really helped me out.
So for repotting, I'd recommend you get a tall container. It doesn't matter if it's over sized, because normal plant rules about being water logged don't apply and it'll fill the pot eventually. I can't remember the exact ratios from the book off the top of my head, but it's something like 70% peat moss, and 30% play sand. The book recommends that you wash it to remove and built up salts or deposits the sand or peat moss may have. I don't know about Spain, but my local big box store has a huge bale of peat for about $7US and a bag of play sand for a kids sandbox is around $3US. You can do a massive amount of pottings with that amount.
The thought process here is that peat is great at holding water and is acidic, which the plants like. They come from a bog area where peat grows and dies and layers on top of itself. There are very little nutrients and that's why they trap insects. So don't fertilize or use miracle grow dirt or anything like that. If you try to "take good care of them" you'll kill them. The play sand loosens the peat to help the roots penetrate. I assume that yours is like most of the store ones and they've grown it either in a peat mixture or sphagnum moss. Either is fine, but I prefer the peat and sand mixture as it works well for me and is cheaper to buy in bulk. I also think the sphagnum moss compacts over time, which the sand helps prevent in the peat moss.
These are super simple plants to care for. I simply pot them in tall pots, mine are ~6" dia and 10-12" tall. I set those pots in drip trays on the steps in full sun and keep the drip trays full of water all the time. If the trays dry out for a day or two while your on vacation, it isn't really a problem. At least not for me, but try to keep them full all the time. That's it. Besides plucking off dead heads or cutting off dead pitchers when you get pitcher plants, that's all that's required. You should only repot CP's like this in the spring, before they come out of their dormancy period. If you have to repot, you can, but everything will be smoother if you wait till next spring. These plants have delicate roots systems, so I wouldn't personally repot them too often, but when you do and if they're dormant, you can just rip the clusters apart with little ill effect.
The reason I use the taller pots is that these plants like to be wet all the time, but they don't like to be swimming. Hence I use a tall pot with a ~1" drip tray full of water. I found that they don't like it if I leave them in the small pots like yours is in, because the drip tray is much taller proportionately and they're too water logged. There is a ton of pot that stays damp because it's wicking from the bottom as it evaporates from the top. The only concern here is that you get a potential build up of minerals from your well water, but you can simply pick the pots up occasional and top water them well to flush them.
I have well water, which mine like fine. If you have chlorine, just let it sit for a day or so. Or catch rain water. That's the best.
You mentioned the heat. I googled it and 33C comes out to 91F. My plants have been in direct sun, in black pots, in over 100F. So you don't have to worry about them up to 38C. There are a couple of things I'll mention about that. Not every day is that hot, but in the middle of the summer it stays in the low to mid 90s (32-35C). We also have a killer amount of humidity. If you have heat, but don't have humidity, you'll have more of an issue and they won't be able to stand it as well, but there are things you can do. I'm not sure what Spain's humidity is like. You should know that the drip tray and the evaporation of the pot will help create a bubble of humidity around the plant. You can also buy or paint the outside of the pot a lighter color and that will help with the heat. You can also move your plant somewhere that it gets a lot of sun in the morning and less at mid-day or shortly after when it's hottest. You can put it in a bigger drip tray, too. If you want to increase the humidity, but a tray of water doesn't create as much humidity as you might thing. It needs surfaces to help it do that. That's why people add stones or sphagnum moss to humidity trays to provide lots of surface area for it to evaporate from and kick humidity into the air.
People do grow them in the house okay, but you'll do much better outside. These plants need full sun to thrive. They also require a winter period or they'll die in a couple of years. They have to have a dormant period. I leave mine outside in the same spot all year. They say it's not good for them, but my pots freeze solid and get snowed in, in the winter and they do well and put out flowers in the spring, so I don't think they care.
Different CP's act differently, like Sarracenias will have pitchers die off over the winter and grow all new ones the next year. I tend to just lop them off, but it's personal preference. You can wait till the spring, because technically, if they still have green on them, they're photosynthesizing. VFT's will actually still put out traps in the winter, but what they do is get compact. Yours is pretty compact right now. It'll really stretch out toward the light and the stems will get longer and the traps will get bigger in the summer. Come winter, they'll get smaller and be much closer to the soil.
As for feeding, if they're outside, don't feed them unless you just want to see it happen and then, not more than a bug a week. They really don't need the bugs that often. Keep in mind they grow in a super pore area and just catch the occasional bug to supplement their minerals they aren't getting from the soil or water. They'll catch plenty. Ironically, while they're called flytraps and people always want to feed them house flys, I've never seen mine catch one. They catch more granddaddy long legs than anything as well as other random bugs.
Good luck with your plant and let me know if I didn't cover anything. Visit the forum: Terra forums if you want to find a lot of good info.
Edit based on further clarification:
I've read here (http://www.sarracenia.com/faq/faq2160.html ) that if
the plant is alive, I'd probably should not repot.
I don't think I'd pot if it wasn't alive. :)
I've checked about the substrate needed, and I am pretty sure I can
get the "ingredients" in Amazon, so it shouldn't be a problem.
I think they're going to kill you on shipping. These two ingredients are common at almost any garden store.
I purchased my plant from Ikea, I don't know in what kind of soil it's
planted , but to me it looks like regular potting soil.
Regular potting soil will quickly kill a CP, so if it looks like potting soil, it's almost certainly Peat moss.
The pot they are currently in is not really tall, so this is why I
think on putting them into water regularly (maybe once or twice a
week, when they need it) instead of let them always in a tray of
Peat moss becomes hydrophobic when it dries out. If you let it dry out too much, then when you water it, it just runs over the top, down the sides, and out the bottom. It's almost impossible to rehydrate, soaking is your best hope, and it prevents water from getting to the roots, which quickly kills it. I think you have two options. One is to get a tiny drip tray for the tiny pot. The second option, which I'd do, is to repot, but don't disturb the roots. Most repotting happens by removing the plant, getting rid of as much of the old soil as possible, and putting it in new potting soil. This is because the new soil will have lots of new nutrients. CP's don't like nutrients. You can mix up new soil, and pot this out of the old pot and right into the new soil without disturbing the roots at all. They'll expand into the new soil. You only want to rip them apart when they're super crowded. They like growing on top of each other.
Tap water where I live is horrible and super hard (lots of calcium
carbonate) and it hardly ever rains, so I think I'll buy distilled
I'm on a well and the water is pretty hard. It leaves calcium deposits on the outside of the pot as the tray water evaporates. It is frequently flushed by rain water. The distilled water isn't a bad idea. Distilled water will is actually so pure it'll pull nutrients out of things so that can rebalance itself. That's why you shouldn't use it on regular plants or drink it yourself. I don't think your tap water will hurt it if you leave it sitting in an open container for a couple of days. Another good resource you have available is rain water. Just get a barrel you can attach to your gutter and have way more than you need. The only caveat is how close to the see you are. If you're getting salt spray on your roof, then it'll be washed into the barrel with your rain water.
I live near the sea, so there is humidity but usually not too much (it
depends on the day, and where the wind is coming from). So I asume
that keeping the plants in my north-facing balcony (making sure the
soil doesn't dry) should work.
I think you'll be okay with the humidity. As long its not a desert I think you'll be fine. They seem pretty tough. Northern exposures in the northern hemisphere have the least light of any other exposure. They need a lot of light. I'd put them in a southern or south eastern exposure if at all possible.
winters are actually quite mild here (temperature rarely drops below
5C/41F). Should I just leave the plant in outside, just watering it
I can't comment on the exact temp they need to drop down to, because I live in their native habitat and have never had to look it up. SC whether is extremely weird. People coming from up north have a horrible time. They're used to it getting cold one day and not warming up till the spring. The saying around here is, "If you don't like the whether in SC, give it 3 days". Some days, the highs are in the 40's and other days their in the 70's. That's when we ride horses. Nights can be anywhere from the high 30's to low 20's and every once in a while gets down in the single digits. I would leave your plant outside. Don't water it less. Keep that drip tray full year round. There is a procedure for putting your plant in the fridge for people keeping them in climates that don't have a winter. Again, I can't comment and you'll have to research it.
I gave the plant a house fly that my cat had just hunted (that's what
I call "team work" haha), but a couple of days later I found out that
the plant had caught a couple of flies by her own. Good work!
That's what I was talking about. You only feed them to see the traps work. They catch stuff easy on their own. Especially once they're outside. Also, my cat's not helpful. He's a b-hole. He vomited a big hairball by my bed the other night and then vomited bile about 18" past so when I stepped over the hairball, I'd step in the bile. I missed it, luckily. Then I arbitrarily decided he'd been rolling in the dirt and needed a bath. Funny how that happens.
I'd pot this whole root ball and soil in a new pot and stick it out in the shade for a couple of weeks then move it to full sun and let it go. Also, I always recommend you never put your hopes on one plant. I see it all the time. You should go see if you can find a couple of more at Ikea and keep one in the house, take care of one outside, and abuse the heck out of the other. You'll feel much more confident when you manage not to kill it and you'll have knowledge of where the boundaries are. If you just have one, then you don't have anything to compare it to and you won't know whether it's doing good or bad. I can almost guarantee that occasional watering and keeping it in the house will kill it eventually. Good luck.