It's tight. Depending on time of day it can take an hour or so to deplane and clear immigration and customs. Then you have to get to Terminal 3. The Skytrain would take up to forty-five minutes by the time you walk and wait. And that's if you get it right the first time! You could always run outside and take a taxi to Terminal 3 but if there's traffic that can take a while also. Then you have to check in at Terminal 3 and go through security there. I usually leave three hours and I've even scheduled more time in between flights if I'm changing terminals.