Hi there,
I managed to do something similar to this back in SO3 with a horse bred for 10F (1M 2F), he managed to win over 8F, 10F, 12F, 14F.
He captured the Guineas, Derby and St. Leger (English Triple Crown) and for his final instructions for the St. Leger I opted with Hold Up, Dont Trail, Challenge Later.
This should give you the best chance of completeing the feat, and I wish you all the very best.
If he makes it, get a picture up. Good luck!