YMQ is a catch-all for airports in the Montreal metropolitan area (LON is used for London, TYO for Tokyo, NYC for New York, etc.).
- YUL is the main international airport (Pierre Eliott Trudeau)
- YMQ is Mirabel Airport (cargo only for the moment, as far as I know)
- YHU Saint-Hubert, mainly for small planes using shorter routes from the province of Quebec
In a flight search engine, YMQ will almost certainly offer flights to YUL alone (assuming you come from outside Quebec).