The following is from the Canadian Thoroughbred Rules & Regulations:

Pick 3, Pick 4, and Pick 5 Scratches

In the event a Pick 3, Pick 4 or Pick 5 ticket designates as a selection a horse which is scratched, excused or determined by the Stewards/Judges to be a non-starter in any of the races comprising the Pick 3, Pick 4 or Pick 5 the favourite, as evidenced by the amount wagered in the win pool at the time of the start of the race in which the scratch occurred, will be substituted for the non-starting selection. In the event that the money bet in the win pool for two or more favourites is identical, the selection with the lowest program number shall be substituted for the non-starting selection.

However, if a scratch occurs in any leg of the Pick 3, Pick 4 or Pick 5 pools before the pool is closed, a ticket including the scratched horse may be cancelled.