Gotcha, read your other message as ending up with only one surviving each out of several tries with either a pair/group/trio.
Your group of three banggais, they're old enough to show pair behavior if there's at least a male and a female. Do you see any of them get super excited with fins flared out, lining up with one in front of the other with the one in front primarily making the 'jerking motions'? That's pair dancing. An established pair will dance many times daily, this will increase as the female becomes gravid and nears egg laying.
If there's none of that yet, I guess still young or possibly all females.. some report their females 'getting along'- others say not so much. Read of one account of two sex unknown banggais getting along fine, until a third was added then one of the two immediately engaged in 'dancing' with the new fish and these went on to be a pair, with the third pretty much left to itself.