Being that the tank is sizeable it probably has a better than usual chance should it be have been in added to a smaller tank (say a 65g?). Particularly if the established clown are on one side of the tank. However, I think it would be generally a bad idea...
Firstly with the two established and near mated, the newcomer would probably be picked on. Maybe if it was significantly smaller, it may be adopted as an ungendered juvie, but I can't confirm that it would happen.
Also, would it be really neccessary to add another clown, when your other 2 are established, healthy and happy?
When you add the new fish, chances are you are like most people and won't QT it... there is a chance it might be sick and get your other clowns sick. Probably won't happen, but why risk it if you have a good thing going, ya know?
Ocellaris's also aren't really THAT mean.. so you MIGHT be able to get away with it.. again.. risk..
All in all, if I was really REALLY set on doing this, I'd get 2 of them, QT them for 4 weeks, reaquascape the tank, carefully acclimatize them to the new tank at night at the beginning of the weekend, leave a night light on, watch them for 2 hours to make sure they are adjusting ok, then wake up 4 hours later and watch them to make sure everything was ok. Watch them for a few days very dilligently. If things are going poorly, ready to remove them back to the QT tank... and pray.
HTH FWIW. Thought you should have a reply..
