This calculator is based on Harville’s Method (Harville, D.A. (1973) Assigning probabilities to the outcomes of multi-entry competitions.).
Harville’s method assumes that horse that won is automatically discounted from placing and the win probabilities of the remaining horses are adjusted back to 100%.
The win probabilities do not determine the probability that horse x finishes in position y.
These are totally unrealistic assumptions and should not be used in betting purposes as such. But do provide rough estimates.

Trifecta Harville Calculator

Select the number of runners (between 3 and 25):

Enter probabilities for each runner (sum must be 100%):

Calculated Harville Probabilities:

Trifecta Combination Probability Implied Decimal Odds