You need to "choose" three toppings, which means you need to do a combination (versus a permutation, which would only be needed if you cared about the order that the toppings are added to the pizza, which you obviously don't care about). The combination formula is n!/(r!(n-r)!).
12!/(12-3)!(3!)= 220 ways to choose toppings.
*know that ! Means 1211109...321
Then multiply that by the 6 cheeses to get 2206 = 1,320 ways. (Each choice of three different toppings can be paired with six different choices for cheese)