What lounge in T3 do you intend to access? All are in the US or international area, which is segregated from domestic flights, I'm not an expert on YYZ but fly there a few times a year, and haven't noticed any domestic lounges. So it is a moot point that WS isn't part of any alliance (also doesn't interline, so while you don't switch terminals, you do need to drag the bags up to departures level to check-in.