I'm looking to use soaker hoses on my tomatoes this year and need a bit over 100 feet to get all the plants. The maximum length I can get one in is 50 feet so I would need three hoses to get everything evenly. Would they work ok if I connected them all together or would it be better to use a 3-way splitter on them? If it helps the hoses I was looking to get are these.
As long as your tap can provide the required pressure to get a decent flow all the way to the end it should work in series, however you will find that running the hoses in series will lead to greater pressure nearer the tap, so I would recommend a splitter to make all 3 hoses have similar pressure and flow characteristics.
Here's my experience with connecting soaker hoses. It pretty much supports @Rory Alsop's advice, but it was too long to fit in a comment:
The soaker hoses I bought recommended a maximum of 150' in series using two 75' hoses. I tried connecting a 75' and a 50' hose for each row I had planted. My plan was to double back at the end of each bed. Each hose had a flow regulating washer that needed to be removed for connections between soaker hoses. (Be sure to leave the first washer in at connection to the leader hose!) I followed all instructions, and the second hose didn't weep for the last 25'. I suspect it was partially due to the slight slope of my yard. My spigot was at the top of the slope, and the hose ran downhill. However, when it doubled back, the end of the hose was at the very top of the hill, which resulted in reduced pressure.
If you're on flat ground with only 100' of hose, I'd try a setup connected in series. Observe your setup in action, and if you notice the end isn't weeping as much as the rest of the length, go for a splitter. It's not a huge hassle to relay the hose, and in series will save you the cost of the splitter. (My brass 4 way splitter was $14).
My experience is similar to others - a parallel set up is better than long single line.
Also - do not mix different kinds of soaker hoses. We bought different kinds and the result was not good. Not sure of the reason for this but I suspect that pressure drops across one section led to starvation of pressure in subsequent.
Have you considered a french drain or collecting rain water? Depending on your location, tap water can be heavily chlorinated and not as helpful to your plants. If you're going to make the effort to install soaker hoses consider installing a water harvesting system.
If you're serious about even water distribution and arent afraid of a DIY weekend you might consider forgoing the soaker hoses and creating your own system with PVC pipes, which you can raise/lower to meet the pressure requirements for each type of plant.