If they are all young (i.e. small) then they are all juveniles or males. One will eventually grow bigger then the other two which means it has morphed into being a female. At that point she will pick one of the others to be a mate. She will harass the one she picks (to dominate him) and seriously attack the other to try and chase it away or kill it.
It's difficult, not impossible, but really rare to keep more than a single pair of clowns in anything but a really big tank.