having a difficult time with getting around tuscany. after arriving from venice in florence and spending 2 additional days there i want to spend 2 days in tuscany before going to rome. i am interested in seeing sienna, san gimisnano,lucca, and pisa.... way too much???......... what about in 3 days? how would i do it ....rent a car and drive? trains and buses ? can i use florence as a base and do day trips?....i am thinking sienna is a full day but can i do lucca nad pisa in the same day on the trains? is san gimisnano a full day?

After Florence I'd take the bus (very easy) to siena and stay in siena. The day you arrive in Sienna , I'd Spend in Sienna. After that I'd rent a car and see the countryside,etc and come back to Sienna in the afternoon or evening. Dinner in Sienna. Evening in Sienna. At the end of your trip to Sienna I'drive to Chiusi or Orvieto, drop off the car, and then take the train to Rome. IMO SG is a half day at most. Other places better to go. Others may disagree.
I'd recommend Montepulciano, and just driving the countryside between montepulciano and sienna. Lots of places within a days drive of Sienna.

You can take a morning train from Florence to Lucca, spend a few hours and leave mid-afternoon to then take a train from Lucca to Pisa (short train trip). After seeing Pisa (Field of Miracles is best seen shortly before sunset), take an evening/night train back to Florence.

For San Gimignano and Siena, with the time you have, I'd spend about 4 hours in San Gimignano and 1 1/2 days in Siena. Not sure if you take trains or buses between Florence and San Gimignano/Siena as I had a rental car. Picking up a rental car in Florence to go to San Gim/Siena before Rome may not be a cost effective move since you would have the car less than 3 days. But, you can always check kemwel.com and autoeurope.com, etc.

Have a wonderful trip! And, if you have the time, try the wine tasting at the convent's wine shop in San Gimignano!

I can't tell you about busses or trains since I'm driving. But you're perfectly right that Siena is a full day (minimum), and it's also possible (though not exactly comfortable) to do Pisa and Lucca on one day. San Gimignano is definitely not one entire day, half a day maximum (MAXIMUM), and as leuk said, not the most interesting of Tuscan destinations, at least not the way most people are doing it. Which is to say that there actually is one terrific sight there, but almost nobody bothers to visit since it's on the edge of town: the Gozzoli frescoes (which are among his very best) in the church of S. Agostino. So for Gozzoli admirers, S. Gimignano is certainly a must. For those traveling the way most people are today (no guidebook, no plan, rush-in-take-one-picture-rush-out for churches that require one hour minimum), it's most probably not worth going, since atmospherically, there are lots of more appealing places. But if you admire Gozzoli, you could do S. Gimignano (half a day), and then go back to Siena, where you may find yourself rushed with just one day.
Anyway, doing everything from just one base is, thinking of a car (again, I don't know anything about public transport), only possible if you're not doing it from Florence - you'd waste way too much time getting out of and back into town. Traffic can be hell in Florence! So what I would do is either choose two locations, Lucca and Siena, and do Pisa and S. Gimignano, respectively, from there; or base myself in either Certaldo or Castelfiorentino, which are delectably untouristy places (and very pretty, as well, above all Certaldo), which are about the only places from where all the towns you want to visit are in easy enough reach.

Florence to Pisa Lucca.
Depending upon what time you plan of leaving Florence and how much of Pisa other than the Leaning Tower that you want to see you can do the following:

Take a train from Florence to Pisa San Rossore station. Make sure the train stops at this very small station right after it stops at Pisa Central. Check before taking this train.

The San Rossore station is a 5 minute walk to the Leaning Tower area. You can wander around there for as long as you like (our only interest there is the Tower so I don't expect to spend much more than an hour there), go back to the same little station and get on the train from there that goes to Lucca. It's only a few minutes ride.

You can spend the rest of the day in Lucca and take an evening train directly back to Florence.

Siena and San Gimignano and quite close to each other and are connected by bus or bus/train connections.

Visit Lucca/Pisa as a day trip from Florence using the train.

Bus from Florence to Siena and spend two nights. Visit San G as a day trip from Siena using the bus. Here is a nice hotel/b&b in central Siena-

http://www.residenzadepoca.it/en/offerte.php

Take the direct Sena bus from Siena to Rome and a taxi from the bus station to your hotel. The bus is very nice and the trip is 3 hours.

Siena/Rome Bus- http://www.sena.it/