In a large enough tank a clown harem can work but in such a small tank there isn't enough territory and the non paired up male gets singled out. You could get lucky and they could all co exist but I wouldn't count on it.
Plus with 3 fish you're close to stocked. Do you really only want 3 clowns?
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"