You'd be unlikely to get through security at T3 to even get close to the MLL. And then the basic stricture, which goes for all Maple Leaf Club members (Elites and Diners] except SE's, is that you must be flying AC and have a valid boarding pass on the day of admission. Since the Diners Club lounge is not associated with any one carrier, AltaFlyer would have been in his rights to clear T1 without a boarding pass. In your case, T3 has extra heavy security and immigration clearances because it deals with overseas flights.

Also, lounge access is generally not available even to members (except SE's) after a flight, only before or connecting onward from an AC flight.