From what I understand, if they are older than 6-9 months, then they can be ready to breed. I am not good at telling the sexes, but if you have 2 females and 1 male, then they could be fighting over the male. Or if you have 1 female and 2 males that are trying to mate, they could be fighting because she doesn't want to.
Maybe if you post some really close pictures of their ceres and tell how old they are (or how long you have had them) then someone else that is good with judging the sexes may be able to help you.
I wish I could be of more help. It sounds like fighting, but I don't know why.