I was working on this one, but gave up in frustration.

the formula i used:

(A x B x C) / (1 - A) / (1 - A - B)

where,
A = winning chance % of 1st
B = winning chance % of 2nd
C = winning chance % of 3rd

After doing the trifecta tables in Excel the file was up to about 39mb in size, which made it almost unusable, prone to freezing etc.

It may be more practical to base your bet size just on the exacta combo's with muliple runners to run 3rd, or some other variation.