Is this the first time you're visiting Tuscany? If it is, then you cannot miss Florence! Are you arriving into Pisa? You can spend a half day there, then continue on to Florence. Spend a whole day in Florence and for your third day.... you could take a tour of Chianti that takes you to San Gimignano as well. You can move around on train so you don't have to worry about renting a car.
Three days will fly by, depending on what you like to see and do will influence what you end up doing in each place