You could easily find enough attractions to spend a week in each of these cities, but you can see the highlights of all three in a well-planned week.

The total travel time between Venice and Rome is only three and a half hours, so you can make the maximum use of your time.
Of course, you’ll want to see the main sights, but do allow some time to stroll through a neighborhood or two and sit in a cafe to soak up the atmosphere.
For example, after you’ve ridden the length of the Grand Canal and toured St. Mark’s Square and the Doge’s Palace, cross the World famous Rialto Bridge into the rabbit warren of narrow lanes in San Polo.
Here, you’ll find mask makers, tiny shops, and artisan studios, as well as churches filled with art and squares where neighborhood children play soccer.
In Rome, after the Colosseum and St. Peter’s, explore the streets of the former ghetto of Trastevere, south of the Vatican.
In Florence, follow your tours of the Pitti Palace art museums and gardens with a stroll through the Oltrarno neighborhood. Longer is better, of course, and with a day or two to spare, you can either see more of each of the “Big Three,” or you could make a stopover to see Bologna or hilltop Siena.