We are renting a car and travelling south stopping in Florence, Sienna and Rome. Are there places to park rental cars in the cities and is it expensive?

If you are really determined to have a car for your trip, then I suggest asking the hotels where you are staying if there are nearby parking lots/garages and what is the cost. And then you can calculate whether it's really worthwhile to have a car that you are paying for just to park it when in these 3 places.
Marie

Are you beginning in Florence and ending in Rome? Most folks find it better to plan their schedule around renting cars, such as flying into Venice, picking up the car on the way out of Venice, driving to and staying in the countryside near Florence and taking the local trains for day trips into the center, and then dropping the car upon arrival in Rome someplace convenient like Borghese Gardens. You can even stay just outside the walls in Siena with your car and train to Florence from there. Hotel parking in Rome and Florence can be brutal and they often charge every time you need access to the car in addition to the daily rate. You are not going to want to drive, in fact often won't be allowed to drive, inside the city center. There are many areas closed to traffic. If you must, Florence has a huge car park at the train station. But in my opinion no visitor should ever drive in Rome, ever, ever, ever, ever, ever, ever. And this coming from a girl who used to cut class and drive into Manhattan the second I got my drivers license. We always rent cars in Italy and drive everywhere. Rome is the only place that I will never drive.