I'm not sure he's suggesting three clowns, but rather a couple of clowns with a banggai and royal gramma.
Still, I'd say be careful. Tomatoes can be aggressive to other fish and your tank isn't huge. Has the current clown established a distinct territory in the tank? If not, it may be easier to introduce other fish. I'd also be tempted to introduce the others before the second clown. If the first hasn't identified a clear territory yet, once you add the second, I assume they will, and in a small tank, that won't leave much room for the other fish.
Hopefully someone with some experience of tomatoes can chime in here.