yellow tang would need a bigger tank.
niger would need upwards around a 75... but if you jsut like triggers in general a undilated would fit.
and now that im re-reading what you wrote, im assuming the clowfish was clownfish and not "cow"fish... cause they will not fit.
But a hawiian blue, snowflake, and a clown will do fine
not too much room besides that though. Another fish, or 2 if you make them small probably.