Three days gives you enough time to see the landmarks without making the entire trip about landmarks. Use the third day to follow your interests.
Day 1: Historic Montreal
Explore Old Montreal and the Old Port. Include one major interior attraction and leave time for the streets themselves.
Day 2: Mountain + neighbourhoods
Visit Mount Royal, then spend the rest of the day in the Plateau and Mile End.
Day 3 option A: food + markets
Start at Jean-Talon Market and Little Italy, or choose Atwater Market, the canal and Saint-Henri.
Day 3 option B: museums
For art, history or poor weather, build around Downtown museums and connected indoor spaces.
Day 3 option C: family
Families can devote the day to Space for Life, the Science Centre or another major child-friendly attraction.