Do they have separate rooms? If not three separated time-out spots. The instant you hear a fight they all got into time-out. No ifs, ands or buts, all of them. They are all old enough to be able to to behave with each other. They have been warned time and time again. Don't put up with it, save your voice.
To preempt the fighting, get them outside! Bundle them up and send them out. If they don't have proper gear for the weather, go buy it. I only have 1 and I suit her up every now and then and send her to the backyard just to get her out of my hair.
Good luck.