I looked into this when I set up my 230g a couple of years ago. From all my research (80% textbooks, 20% internet [Canreef, ReefCentral, etc.] I figured that they all need to be added at or around the same time, and need to be in groups of more then 7-10 (depends on the source). There seemed to be a general concensus that adding 3 will trigger the death of one clown, as one of the three will differentiate into a female and choose one male as a mate. The female would then kill the non-mated male. Granted this was just stuff I've read, and I never did try it out.
However, I have kept 3 separate types of clown pairs in one tank, tomato, ocellaris, and panda. It was just a matter of introducting them at the same time, or rearranging the rockwork/environment upon introduction of a new pair.
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