I got a breeding SnowOnyx with three juviniles in a 100 g cube and did not have problem for more than 1 year (thin I took the tank down and put everybody to the curent DT minus the breedign pair).
Really, Ocellaris and Percula can be keep in group in all my experieces. I got trio of ocellaris since the old days in late 1990's with my Magnifica. The female were laying regularly
Not all clown fish species are the same.
I agree that in a bare 20 gal, one should not try to keep 3 clown fish. Initially there will be abuse, it the smallest cannot hide, and cannot eat then he will not last long at the begining. Also it the pair got to a territory to chase the third fish out, they often just end the chase when the other fish run out. If there is an anemone that they call home, they will be content with just keep the third one out of it. They go back to their anemone when the third fish run away. They don't go to the corner to keep on punish the third fish that hover right there.
It is really important to have these fish healthy, well feed, and have room to go to or hide when the dominate female beating them up at the begining. Maybe I am sucessful because I always have anemones in my tank and often the tank is a full blown busy reef.