What to expect
Day 1: fly from Lima to Cajamarca, keeping plans to arrival and the city. Day 2: Belén museum with free time around it. Day 3: Cumbemayo, returning to the same hotel. Departure from Cajamarca comes after these three days; the plan does not rely on flying straight after the outing.
How it fits into a journey
Start with Lima–Cajamarca and stay in one base. Add the coast or Amazonas only once you have planned the extra travel day.