There needs to be at least 4 selections (legs) to place a Treble.
It consists of individual bets between 3 of the selections for all possible combinations.
At least 3 selections must win to earn a return.
Example
If there are 4 legs selected (A, B, C & D):
Bet 1: A x B x C
Bet 2: A x B x D
Bet 3: B x C x D
Bet 4: A x C x D