Are you sure you're leaving from T3? All AC flights are in T1. If you do have to go to T3, then you will need to pass through customs and go landside. If not, then as rankourabu says, you go through a passport check and continue into the Int'l departures area. There is one PP Lounge there (not great, but OK), and I'm not sure how they police any time limit. If you're in J then you can use the AC Maple Leaf Lounge, which is at the end of the walkway before you go down to the departure area. That is a much better lounge than the PP one, and there are no time limits. I'm not sure if you can purchase an entry if you're not in J.

On the return, you will have to go through customs to get to a hotel. You would have plenty of time to go downtown on the train, or you could stay somewhere near the airport. Going back to the US you will need to go to the Transborder area which is separate from both domestic and Int'l. There are both PP and ML lounges there.