Where the main shut-off usually is
In most Montreal triplexes built before 1960, the municipal water line comes through the front basement wall, usually on the street side, and ends in a valve before the pipe splits toward the three units. It's often in the corner closest to the outside staircase, sometimes behind boxes that have sat there for years.
Some buildings also have a secondary shut-off per unit, higher up, near the ground-floor level or tucked into a storage space. If you can't find the main one quickly, closing your own unit's valve at least limits the damage in your own place while you keep looking.
Recognizing the valve and closing it
Two types dominate: the round wheel valve, closed by turning clockwise until it stops, and the lever-handle ball valve, closed with a quarter turn until the handle sits perpendicular to the pipe. An old wheel valve can be seized, so apply gentle, steady hand pressure without tools rather than forcing it and snapping the stem.
After closing it, open a low tap somewhere in the house (basement or ground floor) to release the pressure still sitting in the pipes. That's what actually stops water still coming out of a pipe that's already split further up the house.
If you can't find the valve
In a multi-unit building, the basement can be locked or shared, and time matters. Close whatever you can reach, your own unit's valve or the supply to the fixture involved (toilet, washing machine, water heater), then call the emergency line: a plumber who knows this building type often knows where to look before they even arrive.
Once you've found it, note where it is, a photo on your phone is enough, so you never lose time over it again.