There are three different choices for the American.
There are 16 different choices for the first non-American and 15 different choices for the other non-American.
So, there are 3 x 16 x 15 different groups.
But, order is important -- who gets the gold, who gets the silver, and who gets the bronze -- there are 6 different ways to arrange the 3 winners (3! = 6).
Thus, there are 6 x 3 x 16 x 15 different results.