I won't say its (3 hour) plenty, but you will be able to complete Insha Allah. The main reason I am telling that the we found the queue very long in the customs where the declaration card will be sealed. Then you need to be headed towards the baggage collection and finally to move to the Immigration for completing the landing formalities.
So, all the queue, delay in baggage collection and the final queue in Immigration are not in your control. Just make sure one thing, if you take any box instead of luggage, and/or any overweight luggage, the those stuffs are coming through belt number 10 (the last one).
What I did for my case, had taken a porter who really guide me through all the way. The porter takes CAD10/luggage and he will finally guide you through until check-in with Air Canada.
Please try to see the following video:
http://www.admtl.com/Site/Gabarits/Globale/FlashPlayer.aspx?fichier=VTS_01_1.f4v&largeur=711&hauteur=475
Also read the paragraph below carefully:
International-Canada
If your flight originated from outside Canada or the United States and you need to take a connecting flight to a Canadian destination, you must clear Canadian customs. It is very important to remember that you must personally pick up your luggage, which you may then leave at the connecting-flight counter located immediately after you leave the customs hall.
You must then follow the normal procedure for Canadian departures and pass through the pre-boarding screening checkpoint “A.” Please note that liquids, gels and creams in containers larger than 100 ml must be carried in your checked luggage. Such items, even if sealed, will not be allowed through pre-boarding screening checkpoints.
Source: http://www.admtl.com/Passengers/Formalities/ConnectingFlight.aspx
Hope these will help.