Florence is all about Renaissance art and architecture. If you love it, 3 full days in Florence is barely enough. If you don't, 3 full days in Florence is a long time.
You don't say when you're going or how long you're planning to be in Rome and Sorrento. If you don't want to wallow in sculpture and painting in Florence, maybe add that night to one of those two destinations.
Another alternative is to spend the 4th night in Venice, leaving early and sightseeing in Verona (one of my favorite places in Italy) or Padua for the day, arriving in Florence in the evening. Store your luggage at the train station for the day.