Add a fourth day if you want a slower pace or a second day trip. Sydney is usually the anchor of a wider Australia trip, so most people spend a few days here before flying north to Cairns and the reef.
How many days do you need in Sydney?
Three days is a good fit for Sydney, one for the harbour (Opera House, Circular Quay, a ferry to Manly), one for the beaches around Bondi and the coastal walk to Coogee, and one for a day trip to the Blue Mountains.