All three of the grasses you've mentioned are cool season grasses - these are best divided in early spring, as they come into growth, so its a little late to do those now. Warm season grasses do not need dividing so often, and are best divided a bit later, since they usually come back into growth later, see link below. Usual method is to dig them up, chop in half and replant, but guidance on that is also given in the link below, though I was amused to see you can 'tear apart clumps of Hakonechloa'... I always take the bread knife to mine, no way is that tearing apart, it gets way too large for that. In fact, I find a good bread knife indispensable when dividing any plant...
I am very familiar with these ornamental grasses. Mexican Feather grass or Stipa t. will need to be controlled. It is a prolific seeder. I love it very much but I used it on a penthouse patio in pots and let me tell you...too much work to control it and removed it. Carex? testacea? Orange Sedge? I love this one bestus. I have never found the need to divide ornamental grasses other than Miscanthus or Maidenhair grass. After 5 years or so. I found I had to control these grasses.
Please send pictures of your grasses, their environment, your zone, how long these grasses have been growing, how close you have planted, let's ID the grasses because each genus and species is different has different needs. If you have any of the 'evergreen' grasses such as Blue Oat Grass, you do NOT want to cut back. Carex and Stipa are like hair; grab a ponytail, add a little twist and cut a few inches off the top, let go and it is beautiful. Try not to allow these grasses to go to seed.
I am not saying you can't or shouldn't divide these grasses. I've grown these grasses where there are winters. That sort of keeps the clumping grasses in check. To freshen my orange sedge for instance, I would simply dig up baby orange sedges, put them in a small pot with potting soil. Then transplant when that plant is larger to freshen my sweeps of this grass in my ornamental landscape bed. The best way to use these is by planting as a 'gang' or group. We can talk about that later.
I hope I have forewarned you. Your only problem will be pulling and thinning these grasses.
You will have baby stipa and carex everywhere. No big deal. These grasses need to be refreshed. They look best young. Old ones are raggedy and even when divided still look raggedy.
You really have to send a picture or two and more information. Ornamental grasses are worth their weight in gold if you know how to use them and manage them.
UPDATE:Maria you have done a fantastic job with these grasses. You have design sense! They don't need any dividing this year. The Stipa needs to be trimmed up when those seed head start forming. I make a pony tail straight up and twist then cut off a few inches. Not too much because they need to have weight to make them arch over to the other plants? Take your fingers and pull them through the grass to pull out the seed heads, try it anyway. These haven't begun to put on the seed. I'd plant these where you don't mind them taking over. Lovely in the breezes.
You can trim the carex the same way. Whatever you are doing with the blue fescue is perfect.
That bronze carex is gorgeous at the nursery. All swirling and glinting in the sun. Bring them home and they look like dead grasses. You should try Orange Sedge. Much more tamed and look a lot like your Mexican feather grass. You will find babies here and there that you can dig up when little and put them in 3" pots with potting soil. When they get larger, plant out in your beds or up pot them to 6" pot until you decide. Keep the pots out of doors where you can water them and they stay acclimated to the sun.
These are #1 or plants from the nursery in 6" pots; lower right corner. The Rhodys are newly pruned but we wanted to save them, all the rest is new. Notice the orange sedge planted on the other side of the walkway on the boulder wall. You want to pull the color into the rest of your landscape, or rather you want to have a few other clumps of the same plant elsewhere to balance and naturalize. Versus using just one gorgeous group. These do the hair cut thing nicely.
Blue fescue is actually tough to use well, and you have done very well. Be careful to not build any mulch or soil on the bark of your trees and shrubs. I can't tell in your picture. I wouldn't do too much with fescue, it is more that each plant seems to have a mind of its own, different sizes, shades.
Bag the bronze, I can already tell you are frustrated with it. I did the same thing in a client's landscape and ended up pulling them all out. They just look like dead plants.
I think you'll like the Orange sedge. Try a few Maidenhair grasses. They get big but in the spring early spring you cut them all the way to 6" above the ground. The fountain grasses are a nice medium height grass that have lovely seed heads all winter long. Then you cut them all the way back in early spring, too. Do that with the stipa and the sedges.
Evergreen grasses you do not cut back and Blue Fescue is one of them. I am adding an article that is helpful. The line in the article saying this grass doesn't need fertilizer is bogus. Compost is not fertilizer. I used Osmocote 14-14-14 in my landscapes. It is slow release and lasts for 6 months. I threw it on my beds in spring and that was it. Usually before I put down fresh mulch.
Down further in the article it tells you have to divide your fescue. I divide anything with a shovel. Bread knife won't hack it for me. A thin hand saw is made for this very thing. Any thin or narrow saw about 6-10" long will work best. I had to lay my big pots on their sides and pull out the entire potted plants with soil at the end of a season and then saw off chunks to remove the perennials from the annuals that go into the compost bin or into the debris trailer. The plants I wanted to save to use the next season are transplanted into pots to be stored in the greenhouse for the winter (roots are susceptible to cold when in pots) or planted into the landscape. I can't imagine using a bread knife, sorry Bamboo. Grins...bread knives will work for smaller jobs of course! But serration to get through root masses is actually gentler than levering the plant into sections.
You say your area is zone 10? Cool season grass lawn? Looks more like a 5? Where is it you live, Maria?