Fighting finches =[[ need help quick!!?

ok heres the story...i originally had a boy and girl zebra finch...the girl died =[ from egg binding..I went to the petstore and the man said that I should get another boy and that they never fight with eachother...so I had 2 boys...then today my dad bought me another finch..another boy and he looked like he was picked on in the petstore due to the loss of feathers...look like they were plucked off..so i now have 3 boy finches in a nice sized cage...and the newest one is trying to pick at my other 2.....(which were perfectly fine with eachother before this..) the new one chases the other to around and makes their little lives miserable =[ I was going to put him in a cage alone..but I heard that they must be atleast with 1 other finch..can anyone tell me why they are fighting?? and how to make them stop? PLEASE???