I wonder if you're an Elite or higher, can you check in at T3, take the shuttle to T2 and use the lounge for a couple of hours before heading back over to T3 to board your flight. Then again, you'll need to arrive pretty early to do this.
Or if you don't have any bags, can you just check in using the kiosk at T2 and then use the MLL for a bit before heading over to T3.
[I'm talking about YYZ]