Your bags will have to clear customs at Toronto somehow. That is without question. It has to happen at the first port of entry, because baggage claims for domestic flights in Canada (and pretty much everywhere in the world outside Europe, to be honest) are separate from those used for international flights and are not equipped for customs checks at all.

The question is whether YYZ T3 is equipped with Canada's new connections customs system, where the CBSA officer shows you a picture of your bags on screen and asks to confirm if they're yours. Then they can either choose to have them sent through to your next flight, or call them to the baggage claim if they want to inspect. This alleviates the need to pick up and recheck the bags in most cases. Last time I went through YYZ a few years ago, T3 did NOT have this system - only T1 did. So international -> domestic connections at T3 had to pick up and recheck bags in all cases. But it sounds from Stanbury's post above that they may have changed. If they do have the new customs systems, I would think they should work with any airline combination.

I think this question would probably better be asked in the Westjet or Canada forums as they will have more folks more familiar with the latest conditions and connection procedures at the airports there.