miss Z, in you case I think a major factor was that you had 3 females together. this is fine for males, but you should never house more then 2 females together. there are exeptions of coarse, I once had a cage with 5 females together and they were totally fine, but that was an exeption to the rule. I house my females very carefully according to temperments and age.

the right temperment is key here, when you choose a cagemate for a dominant or aggressive female, you want a larger cagemate who is calm and good at giving calming signals. you know how some dogs just get along with all dogs, no matter what aggression problems they have? its because they know how to use their body language to diffuse the situation, as opposed to getting mad back, or getting scared, and just aggravating the issue. its the same with Gerbils, some Gerbils are simpley very good ad readng other Gerbils, and diffusing situations. these gerbils make GREAT companions for dominant or aggressive gerbils.