Take it out of the display tank and put it in a hospital tank. If you don't have a hospital tank or a quarantine tank ready, use a spare salt bucket with a heater and a power head. Don't put it back into DT regardless of the outcome (reason below).
Treat it with saltwater maracyn-2.
It's not advised to put more than one bangaii cardinal in a tank unless they are a mated pair. I bet you that the other two did that to the little guy.
Good luck!
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