As titled, I don't see anything said which terminal on the itinerary from Expedia, all I know is the time, the airline Transat and airport is YYZ.
But there is a terminal 1 and terminal 3, which one is for going to Rome?
There is no Terminal 2. Terminals are assigned by airline, not by destination. There are divisions within each terminal for destination, but you start at the desk for your airline. Transat uses Terminal 3.
You can get this information for any airline from the YYZ web site.
If you are being driven to the airport by a paid professional (taxi, shuttle bus etc), don't tell them to take you to Terminal 3. They might drop you at the wrong end or have to ask you more questions. Instead, tell them "Air Transat International" and they will take you to just the right place.