The good news is that the STM launched a new and improved airport service earlier this year, the 747:
http://stm.info/English/bus/planibus/747.pdf
The bad news is that you can't buy the 3 day tourist pass on board. The $7 single fare, however, is basically a one-day pass for the entire STM bus and métro network (note: coins only and no change given, so get loonies and toonies as soon you get through arrivals). Perhaps you can work some kind of combo of the one day pass to get you into the city and then a three day pass for the remainder of your trip?
Montréal blogger Steve Faguy explained the new service earlier this year here:
http://blog.fagstein.com/2010/03/11/stm-747-airport-express/
Although I've not used the service yet, it looks like a big improvement on the overpriced bus service that used to be provided between the airport at the central bus station. It could be cheaper, but a $7 day pass is definitely more attractive than the previous cheap alternative, which was to take an half-hourly local service from the airport to Dorval train station to connect to an express... not the most convenient or user friendly, especially in winter time.