The other posters are correct in that there is no duty-free shopping in Terminal 3. If you allow yourself enough time, I have had passengers travelling on our flights to Calgary and Edmonton go to the International Terminal (they've been let through security if you mention you're going to purchase duty free for a Canadian flight leaving from Terminal 3) and purchase duty-free items there, and they have been delivered to the gate by duty free personnel. But it hasn't happened very often, mainly because of the lead time you would need.

YMMV. I can't guarantee this will work.

Happy Travels!