Barbara,
You didn't say if this was your first visit to Florence, but that Itinerary sounds FAR too rushed and busy! As another post mentioned, you could easily find lots to do in Florence for three days.
My suggestion would be to base in Florence and then take a day trip to one of the other locations (Siena is an easy trip by Bus). With your original plan, you'd be seeing most of the area through a windshield and wouldn't really be spending enough time to enjoy each of the locations.
I agree with previous posts - with such a short time frame, I'd drop Pisa off the list immediately. It's not worth it just for a short visit to the Leaning Tower (if you had more time, that would be different).
Happy travels!