If the one other horse is an out price-- which it has to be or the odds of the first three will be longer-- the tri will pay only slightly more than the 12-1 in that race. There will be six for the third slot instead of five, with that one being many many times the price of the actual third finisher. Might add 10%, tops. So instead of the $25 I predicted you might get $29.
Look-- let\'s say AP gets 50% of the action, the other two 15 each. That leaves 20 for the rest. So for the third spot, 65 (50+15) is already out of play because they finished first and second. For the third spot it\'s 15 to win 20, odds of 20/15, on what the exacta pays.
The AVERAGE exacta if AP pays $3 is 3x7, $21. With the overwhelming second and third choices, maybe $10. If so tri comes out 20/15 times $10, plus $10.