In Terminal 3 there is no 'sterile' international departures area. You are free to walk out of the terminal without any checks from the departure gate areas, as well as board a domestic flight. Therefore, all connections with international arrivals in T3 go through the full immigration, customs and security re-screening process. So yes, to get to your gate you are entering Canada and certain nationals will need an entry or temporary resident visa to transit YYZ (in Terminal 3 at least).

That being said, most international widebody flights leave from a restricted entry zone the end of the C pier and there's a boarding pass check, but this has to do with purchasing Duty Free.