You are right Tom. Clownfish are protandrous hermaphrodites. They start off unsexed or as submissive males and go up the "chain" as needed. They can't go back though. Once a female, always a female.
I wouldn't try a third clownfish in a tank. Even in a large system, they can't get away from the pair if needed. An 8 or 10 foot tank could probably work, and a 6 foot may, but a 4 foot tank would probably mean death for the clown. Skunks and ocellaris may be fine, but all the other clowns can wreak terror on the odd man out.