Hi, are they already bonded? I havent heard of three males living together?
I would definitely start with getting them neutered first, then maybe get some advice from others on here about bonding three neutered males, as I am not sure how tricky that would be?
Sorry I am not very experienced with bonding so hope someone else with more experience comes along or maybe start a new thread.
As for litter training, most of ours have either arrived already litter trained or have got the hang pretty easily...just put the litter tray in a corner with some used litter with their poo and wee on it, put that in it so it has their scent in it. Any accidents, mop up and put in the litter tray. Put hay in it to encourage them to go. They prefer corners and preferably put one litter tray per bun. Or you can see where they naturally want to wee and put the litter trays there.
I have just learnt that not all bunnies can be litter trained, but most do. Good luck.