The Priority Pass app is the best and easiest way to find their lounges. It lists them by terminal, and shows which are for domestic vs transborder vs international.

The YYZ T3 lounge is a “Plaza Premium” lounge, near gate A10.

You likely won’t even need to use your Priority Pass for access. (Most) Amex Platinum cardholders have access to those lounges in Canada without having to use their PP account. (Amex Canada kinda treats them like a Centurion program, although the lounges aren’t as good as Centurion.)