The formula Ubetido put in is correct i have used it before works a treat, divis or normaly higher than the rated prices i will give an exaample from a race today.

The Oaks

Win div of the first 3 places from Unitab

1st $2.50 =a
2nd $2.70 =b
3rd $11.60 =c

take 1 from each price
1.5
1.7
10.6

(1.5x((1.5x1.7)-1)xC)/a)

Payout is $10.79

Trifecta payed $34.10