Aloha Duy,
If they are indeed sexually paired they might ostracize the new one. It will hang in a corner away from the pair. One day it will simply be gone. I suggest starting the new one in an acclimation container.
If they are not sexually paired then this will be the opportunity for one of them to choose the new one as its mate and the other will be ostracized and may eventually disappear/die.
If ostracizing happens, it's best to remove the odd Bangaii.
The way to tell if two Bangaii are sexually paired/pairing is that they hang together most of the time. Though they may argue/chase for a few inches, they don't go very far away from each other for long.
Dion may have other/different, but helpful ideas, since he raised a lot of Bangaii.
Mark