In the trixie system, you bet on the 3 doubles and the 1 3-fold resulting from your selections to a total of 4 bets.
To calculate the winnings of each bet, multiply the stake of each individual bet and not your overall stake amount.
For instance, if your total stake is 20€, the stake of each individual bet will be 5€ (5 x 4 bets = 20€), so you can calculate the amount accordingly.