I will be with a tour group in Southern Italy for 11 days
and when we get back to Rome, I have 3 days on my own.
I want to go to Siena, Assisi and Florence. I had wanted to go to Venice but I'm not sure there is enough time.I have May 30, 31 and June 1 for sightseeing on my own.
Anne, If you go to the DeutschBahn site: http://reiseauskunft.bahn.de/bin/query.exe/en
you'll be able to determine the travel time involved between each of these cities and add in an hour or so here and there to arrive at the train stations on time. If I were planning a trip with 3 extra days, I might add one town, two at the most, depending on the travel time necessary!
If you try to see those three cities you'll be spending all your time traveling between them. Better to stay in Rome and explore it, with maybe a day trip out of the city. Save Siena, Assisi, Florence and Venice for another trip. (Or do a daytrip into Florence from Rome on an express train.)
Anyway, that's what I would suggest.
I would agree with Darcy, these destinations are not really close together...If I were you I would go with just Florence and Siena or Assisi and Florence. Don't pack in too much, you'll be exhausted and unable to truly appreciate these wonderful places.