Just be like "Yo dude you piss on the seat and don't flush, you're gross" and he will stop. If he doesn't, piss in his bed.
I think you have to go into shared living arrangements with the knowledge that there will be problems even if your living with your best friend. I know one group of girls once a month has a meeting to air any grievances and for the most part it works. You may want to try that route or talk to each one individually and don't forget you may find out your doing things that are pissing off them. Try to remain calm and try to focus on the issue rather then attacking them directly.
Another tactic if get the landlord or the school involved if your living in residence. I know many of my problems got solved when they were evicted and I wasn't even the one doing the complaining.
I'm also assuming your done in late April/early May? You may want to tough it out for a couple of months and find better accommodations in the fall.
Pro tip: Find out their mothers number. Call her on behalf of the school. Tell their mothers what they're doing wrong and tell them your roommates doesn't care or listen to their rules. Tell them they might get expelled.
There we go, their mothers will fix the problem for you.
You punch em on the right eye. If they get mad, you punch em on the left eye. Problem solved.
Tell each of them these things, firmly?
I doubt they're doing this with the sole purpose of annoying you. It might not fix everything, but you might get one or two of them to shape up.
You don't have to flip out at them, but they won't stop unless they know they're doing something wrong.
If there friends have a word with them, they cant be that good of friends if you cant approach them, no point tip toeing around it.
I'm gonna let 'em know that Dolemite is back on the scene! I'm gonna let 'em know that Dolemite is my name, and fuckin' up motherfuckers is my game!