1) the 1st friend can go in any of the 5 teams, 2nd one can go in any of the rest 4 and 3rd friend can go in any of the rest 3 teams. The other 17 players can occupy the rest 17 places in 17! ways. On the other hand, without any restriction, the 20 people could be allotted in 20! ways.
You should be able to find out the probability now.
2) Where are you stuck at? And what does "choosing method" mean?