Two nights in Venice as your first stop is really too short. On your first day, many of you (perhaps all) will be sleep-deprived and jetlagged. You may struggle to stay away. That day shouldn't really be counted as a sightseeing day, though some of you may be alert enough to enjoy walking around outdoors--which is the best thing to do, for getting over jetlag.
Edited to add: If you have only one real sightseeing day in Venice, you'll probably spend all your time marching shoulder-to-shoulder with thousands of other visitors who want to see the same things you do. You need more time so you can get to the less-visited parts of the city.
I love Venice, but I wouldn't bother to go there for two nights at the beginning of a short trip like this.
You are also rather short in allowing only 2 or 3 nights in Florence. That's just 1.5 or 2.5 days. Most people going to Florence also want to see some of the Tuscan countryside.
I think 3 nights is too short in Rome and would consider 4 the bare minimum.
Unless you can extend your trip, my recommendation is that you pick 2 of the 3 cities.