It sounds like introducing the new cat caused your existing cats stress, and the fighting is just their way of dealing with it. Cats like routine and sometimes all it takes is one thing to throw off the balance that they're used to. It's especially hard with cats that aren't kittens anymore. From what you describe, you're doing everything right. Just try to keep their routine, feeding, play, etc., very similar to what they are used to, and that should help to reassure them. I don't know if you plan to bring the new cat back or not, but if so, try to do the same with him or her (a slow introduction and then a routine). Patience and time are usually the two best solutions for dealing with cats that aren't getting along well.