The Trap is that A can win any of the prizes, it does not say that B will win AFTER A has won, so in this case A can win the 3rd prize as well. We have 3 spots:
_ _ _
Let's take the case where A wins:
A can take any of the 3 spots (including the 3rd) = 3
If A has won, B must win too, B can take any of the 2 remaining spots: 2
The last spot can be taken by any of the 4 remaining players: 4
Number of combinations in which A is a winner = 3*2*4 = 24
Now, let us count the number of cases in which A does not win. Notice here that B will surely win if A does but B's win is not contingent on A winning, that is, it is not a necessary condition for B to win, he can win without A winning but if A does win, B will win too, if A does not win, B may or may not win.
Again, we have 3 spots:
_ _ _
The first spot can be taken by any player except A = 5
The second spot can be taken by any of the remaining players = 4
The third spot can be taken by any of the remaining players = 3
Total = 5*4*3 = 60
Total cases = Cases where A wins + Cases where A does not win = 24 + 60 = 84