if they are the same species, you can keep/add as many for the colony you want, They all start as males, and the biggest one will develop into a female. If that one dies or is removed, the next biggest one will become a female. If you have clowns beating each other up, you may have 2 females, and they do not tolerate each other. When I buy clowns, always take small juvenile males, that are housed together in a giant school. They probably have been together at the aquaculture since birth. If you ever see a lone, midsided clown, there is a chance it has already morphed into a female. (designer clowns may have been separated in their display tanks too long)
In the wild, the colony recruits young juveniles males naturally, so adding new clowns to your tank is the same idea.
Are your 3 clowns the same species and did you buy them at different times? Some species ARE more aggressive then others, (ie, maroon, tomato, clarkii).
I personally keep them as pairs, or groups of 6. If you like clowns and they are the same species, I would have no problem adding 4 to the 3 you have now, without worrying about moving the landscape around.