3 days is absolutely too short time for such a magnificient city. But sometimes you just do not have another choice. Reccomended itinerary by days (assuming you arrive before noon and depart in the afternoon):
- Arrival day: Westminster and Buckingham palace
- Sherlock Holmes museum, Trafalgar square, City of London (enjoy the cheapest overview of London from the Monument)
- Departure day: Southbank with Tate museum