We did exactly this a few years ago, and I completely concur! We split 14 days across Rome/Florence/Venice, and traveled by train in between. If we had it to do over, we would shorten Venice and lengthen Florence. (Spent 7 days in Rome, 4 in Florence, and 3 in Venice on that trip, and would try to take one day from each of Rome and Venice and add them to Florence.)