First thing I thought too Franky.... how long have they been together?
My guess is that the breeder you bought them from pulled older females from one tub, and an older male from another tub, and sold them together as a trio....
Generaly speaking, you can't do that. I have found that I can only put babies from different parents together at a certain age without fighting. That is from just weined (as soon as their eyes are all the way open) to about 10 days after just weined. Any older then that and they will often end up killing each other.
An interesting note... one time when I put a different line older male in with 3 older female siblings, they initially fought, but then appeared to work out their differences. That was until the girls killed him a few weeks later!