Whether to take car or train depends on what you want to do. If you want primarily to see Florence, Siena and Pisa, then I'd take the train--base in Florence, day trips to Siena and Pisa--they're both worth seeing but a day is enough for each. And you don't really have time to do much more than that: Florence will demand the rest of your time,particularly if you're into art.

Thanks for the reply. Were mainly into the architecture rather than art but we will be visiting some of the galleries. We will look into using Florence as a base and see what day trips we can find to explore the regions around.

you can do a lot from Florence by way of day trips, but if you want to explore in more depth, a car is probably what you'll need.

one way to approach it would be to forget abut Rome altogether and get a train to somewhere you'd like to spend a night or two [? Orvieto?] and then pick up a car, and drive up though Tuscany.

then return the car somewhere outside Florence [? Siena] and get the bus/train to Florence where you will NOT need a car. spend a few nights in florence, then get the train to Venice and fly home.

you can day trip to lots of places from Florence - Siena, San Gim, & Fiesole by bus, Pisa, Lucca, and Bologna by train.

It is alot to fit in for 7 days, as you will probably be traveling almost every day. Instead I would try to cover less ground, pick what you really want to see and not try to pack a 2 week trip into a 1 week trip. If you are going to spend 1 night in both Rome and Venice, then pick one other place, Florence/Siena or the coast (Cinque Terre or Santa Margherita Ligure)and schedule 1 day trip from there. You easily spend the full 7 days in Rome, Venice, or Florence, so if you have seen some of these cities before I would travel directly from the airport to a new destination and end in Venice.
Happy Travels!