I have a trio of Onyx clowns. But before you get all excited let me explain.
I started with 5 juveniles, all from the same batch, all the same age, all sexless and added them to a 300g tank.
Took about 2-3 months before they were large enough to start developing sexes. Im guessing one turned female and 3 turned male because thats when the fighting started.
Two of them would chase each other in circles literally non stop for DAYS, until I removed both and sold them to different people.
I now have a pair and an Juvi clown. The pair tolerates the Juvi, but will chase him away if it gets close to the anemone. He gets in there sometimes, but always gets chased away and goes back to his spot in the flowerpot.
They have existed that way for about 6 years now.
Get a pair and save yourself some heart ache.