три визита к стоматологу = three visits to the dentist He means that the girl had gone to the dentist three times. As Max noticed, the guy is just kidding. He means that "Her smile (or just a smile in general, could be a bit ambiguous) = three visits to the dentist + lipstick". Sort of a formula of a charming smile (ironic)

The second man is ironic. He says making that smile is a piece of cake. She just went to the dentist three times and put lipstick on.