First, I assume you know the "fundamental theorem of counting"- if you can choose A in m ways and choose B in n ways- and your choice for A does not affect your choice for B- then you can choose A and B in mn different ways, the product of the two numbers.

1) There are 19 different types of icecream for the first scoop. Since you cannot use that again for the second, there are 18 different types for the second scoop. Since you cannot use either of the first two kinds of ice cream again, you have 17 choices for the third scoop.