I started with 5 of them and ended up with 2. The big ones chased the smaller ones and probably killed them. I wanted a school, but I'm afraid to put more in. I think they are aggressive fish.
Sounds like you got stuck with 3 males or 3 females. They will kill each other if there is more than one of the same sex. But where you have such a large tank you would think they could escape each other and even more so when there was only 2 left. This is what makes me think there may have been a other cause of death