Why won't dwarf hamsters stop fighting?

i have 3 hamsters in one cage 2 won't stop fighting.i know that dwarf hamsters are meant to live in groups(they have been together a long time and get one fine) . one of the fighting hamsters ( the one that is fighting ) stands on the other while the other squeaks i put the fighting hamster in another cage with the other one that i haven't talked about (they still get on fine) i try puting the other one in after i while put they still fight what should i do now ?