Always consider that less may be more.
Less means fewer travel days where hours are spent getting from one place to another.
Less means fewer hotel check-ins and check outs.
Less means you get to spend more time in a place to relax, or see more sights, or get a feel for the ebb and flow. To hit the same gelato stand a second time, or compare different ones.
Spend some time with travel books. Not just R/S, but DK Travel Guides that really show you what neighbourhood or certain sights or city walks might look like, Moon Travel and Lonely Planet.
As others have said, consider what you want to see and do. Churches? Museums? Shopping? Food? The Passegiata? Music recitals? (Remember that churches in Rome, Florence and Venice have frescoes and statues by the great artists of the Renaissance - Michelangelo, Fra Angelico, Botticelli, Donatello, Ghirlandaio (who taught Michelangelo a thing or two about frescoes), Titian, Bernini and many more.)
Just 10 days. I’d recommend just two main stops and then perhaps a single day trip, e.g., from Florence to Pisa or Siena or a wine tour in Chianti?
You could do the holy trinity of Venice, Florence and Rome. But you would end up missing so much.
If your hearts are set on all three, do four or five nights (3 or 4 days) in Rome, four or five nights (3 or 4 days) in Florence and two nights (one day) in Venice.
Better yet, if you add just two days then you can do five nights in Rome, five in Florence and three in Venice. That gives you 10 full days to spend in the three cities, 4+4+2. And your trip still runs only two weeks from home and back home.
LATE NOTE. While the train from Rome to Florence might be less than two hours, once you consider packing up, checking out, getting to the train station and waiting for the train, and then the reverse at your arrival destination, you have spent 4 hours or more moving about and not enjoying being somewhere. It’s a bit of a bite out of the day. If you do this from Venice, and then from Florence, and then from Rome or the Amalfi Coast, ie three times, you’ve lost a full day. Hence my suggestion to consider doing that just once so you can focus on two main destinations. BTW, if Venice is one of the two, not only is Murano a good 1/2 day trip, Padova (Padua) is also a nice day trip, just about 45 minutes from Venice by train.