IIRC, bangai young gather in longspine urchins in the wild, using the urchins as protection until they are bigger, and of an age to pair off and start defending territory...
Perhaps yours are practicing the same sort of psuedo-hosting?
With three of them, if you are lucky, you will have a pair in the making. Be on the alert as they grow though; usually, once two of them pair off, the third will need to be removed quite promptly, as the social stress can kill it within a day or two.
Edit: If your tank is large enough to give the pair a 3-4 cubic ft range to defend, and leave room for the loner to swim outside that, s/he may be just fine. I just wanted you to be aware
Edit again, lol: I just read your sig, and see that you are breeding the bangai's yourself? You should try making fake urchins for them out of pvc end caps and some stiff plastic threads. Maybe even krylon paint the whole thing black, if you make one. I read about them being used in rearing tanks to provide a sense of shelter, and that the survival rates of the young might even be higher.