The Air Canada 3 man operation YVR-HKG is not always a 3 man crew. The crew compliment is based on the departure time from YVR and the flight time on the day. If it is a late night departure or if the flight time is over 13:55 it will be a 4 man crew. The 4th man PXs home though in almost all cases, so a 3 man crew HKG-YVR is the rule.
Been there, done that, and the 3 man flights are brutal.