First, 3 clowns might not work together. It's usually 1 or a pair (same type). For 3 fish I agree, get a 29g or larger. A pair of percs (nemo) are probably OK in a 15, but the price difference is so minimal...
hey half the fun is dreaming it up!
go for the 29 gal. Agree with srgetz, DON'T get 3. get 1 large and 1 small, this will make all good for the 2 to pair up. With 3, 2 will likely pair up and the other will be unwelcome, especially in a smaller tank.
Good advice on both the tank size (30gal) and not putting 3 clowns together. When selecting a pair, its ok to get one a little bigger than the other. If they are all roughly the same size, that's ok too as long as they are still pretty small. One of them will become the male adn the other will be the female. Percula clowns have the ability to change gender prior to full adulthood. That's why people almost always end up with a paired couple.
Former advisor and planted tank geek...life's moved on though.