Yes, it is required. But you won't need to haul your luggage with you to T3.

On the Pearson website there is a connecting guide. Enter your arrival and departure information and it gives you a step by step guide on what you have to do.

http://www.torontopearson.com/Connecting.aspx#

Note that the train is currently out of service until the end of October and shuttle buses are running between terminals.