Hi, Jasmine: First off, you need to tell us where in Tuscany you will be staying. Given that you only have three days, you want to maximize your time in the towns and minimize your driving time. Second, I wouldn't try to visit as many towns as possible, check-list style, but try to truly experience each town...spend some time amongst the locals.
Your third day looks fine to me. On another day, I'd visit Pienza and Montepulciano...they're not very far apart and quite nice.
Depending upon where you're staying, you could do Cortona and Assisi in a day (although you'd be venturing outside of Tuscany proper).
Finally, Tuscany has so many charming tiny villages, such as San Quirico, Castlemuzio, and Monticiello, that you may want to just drive, take in the scenery, and stop and explore without a set itinerary.
In any case, have a great time!