Many factors in the possibility of adding a new clowns. Assuming your new fish is not similar size as the old ones and if you have a separate temp hospital tank, you can make the original feel like they are the new guys.
1)Prepare your hospital tank ready to QT you new fish
2)At the ending day of the QT, drip acclimate it to the new tank water.
3)Catch the Old clowns and put them in the hospital tank.
4)Rearrange the aquascape and put the new fish in the DT
5)5 days later, put 1 of the 2 old fish from the hospital into the DT.
6)Fighting should be minimal. If it turns very violent then trade the fish away.
7)3 days later introduce the last one back into the DT
I am pretty sure the two old fish will recognize each other but much less likely to gang up on the new guy.
I frequently move clownfish from one tank to another and realized they can remember their friends and mates if the separation are not too long.
Last week, I just move 2 Platinums that lived together for ~2 years into a smaller cubicle with 2 Sanjay fish plus one more platinum. Because all 5 fish feel like they are the new guys, none of them show any aggression towards each other even though in the old 60G, the two old Platinums will fight a little once awhile.