How many days for these cities

That would depend to some degree on your interests. (Ancient ruins = more time in Rome.) And what sort of environment do you prefer: big, vibrant city or something smaller? Do you have plans for any day trips?

May I suggest you look at the RS (or others) guidebooks and make a list of what you want to see, where you want to eat (hey, it's Italy) both during the day and at night. That might help you focus a bit. The three cities you mention will be very hot, very humid, very crowded. That means you'll be moving slower because of the crowds, the humidity and the heat. Many of the major sites and museums have time specific reservations. I would suggest taking advantage of that. It will keep you away from some very long lines while standing in the hot sun surrounded by some very sweaty people. Gelato and lots of water, sun screen, comfortable clothes will take care of those negatives. Give as much time to each of the cities as you can. The suggested list will help you manage your time. Reservations will increase your time doing what you want to do. See what happens to your planning with this schedule: 4 nights Venice (that's 3 days); train to Florence for 3 nights (that's 2 days); and the rest of the time in Rome. I give more to Rome because Rome has so much to offer, including possible day trips to Ostia Antica, Orvieto. Rome also has many more centuries and cultures to explore than the other two cities. Hmmm, why so much time in Bellagio? It is a nice place to get over jetlag.

Is that 12 DAYS or 12 NIGHTS?

Where are you flying home from? assume you are flying IN to Milan?

Actually, I think 12 days for those 3 cities is fine; we've seen much shorter itineraries! Without knowing what you're most interested in doing/seeing (what are those things?) I might split them up like this, and in this order (1/2 day subtracted for each transfer day)

Venice: 3 nights/2.5 days
Florence: 5 nights/4.5 days (allows time for a day trip or two)
Rome: 4 nights/3.5 days

OR......

Venice: 3 nights/2.5 days
Florence: 4 nights/3.5 days
Rome: 5 nights/4.5 days

It would be helpful to know who "we" are?

I agree with Kathy on her time recommendations. I would give Florence the extra day, but I love Florence. Either way works fine. No matter what, you will want to return. Have a great vacation.

We like a little of everything - museums, ruins, shopping. In Florence we thought we might do a day trip, possibly including a cooking class. Rome we want to see Ostia Antica. I love seeing gardens in different places. There are 4 of us teen - 50’s. All active and we’ve done other trips and did lots of walking including trips where it was very hot (Greece in the summer).

We purposely decided to enjoy Bellagio while getting over jetlag and it seems like a relaxing way to start off a trip that we know will be pretty busy in other cities.

We are flying into Milan and plan to fly out of Rome.

We had planned for this trip in summer of 2020 but like so many other people that didn’t happen. The amount of days for this trip isn’t set quite yet - thinking total of about 16 days (including the 4 nights in Bellagio). We can add a night or two if we need to.

Thanks Kathy - I didn’t see your reply until after I just posed mine.

Catherine, you could very easily add time to ANY of the 3 cities without running out of things to see PLUS be able to sightsee at a more leisurely pace. Like i'd said before, we've seen itineraries so short that it meant trying to cram in JUST the biggies at breakneck speed! It's refreshing to see a poster who has allocated enough time to learn to get to know the "Holy Trinity" cities. 👍

And I love Florence too.

Having been to those cities more than once in past decade (and planning trip #5 for March!), my best piece of advice is: make reservations for your top choices in each location, BUT don't overbook such that no day has any 'spare' time. Our best memories have been the days we just wander and make our own discoveries. Although of course, seeing David in Florence or the Colosseum in Rome were a 'must do'! But we keep leaving places for the next trip ha ha.

Back in 2010 we did your trip in reverse except we replaced Bellagio with Varenna. I'd stay in Venice for 3 nights, Florence for 5 with a day trip or 2 (which we didn't do), and 4 nights in Rome. If you only do 1 day trip from Florence then subtract a night and add it to Rome.

Catherine, since you like gardens, here are some of my favorites.

Bellagio. Villa Serbelloni.

Tremezzo, short ferry ride from Bellagio
Villa Carlotta.

Venice. Biennale Garden.

Florence. Bardini garden. With more time
Boboli Gardens.

Rome. Borghese Gardens starting at the Pinico entrance.
Small garden on the Aventine Hill Garden Arancia. Great views of Rome.

For a Day trip, this one is fantastic. Short train ride from Rome to Tivoli. 50 minutes, Villa d’ Este.

Astica Antica is not a garden, but the ruins of Rome s old port.

Kathy has given you a great itinerary.

Good list from Gerri. Just as followup, some of these gardens don't mean neatly tended plots of flowers. In fact, in July you may see few flowers at some of them. At the Boboli Gardens, the lawns could be pretty brown, the hedges sort of ragged, and the water in the fountains, if they're even running, pretty stagnant. You might think of the Boboli more as a very large park than a garden? Reviews on Trip Advisor are full of negative reviews by visitors with a different idea of what "garden" means but it's fine if you know what to expect.

https://www.visitflorence.com/florence-museums/boboli-gardens.html

Villa Borghese in Rome (NOT Galleria Borghese, the wonderful art museum; reviewers mix them up all the timer) is similar in some ways; a huge park with lots of trees and lawns, gravel paths, some fountains, a pond, some sculptures but few flowers. I like this one, though, as it's breathing space in a busy city.

If you want a beautiful garden with a bazillion fountains (who counts when there are so many?) including a ship fountain and a playing organ fountain and fabulous green spaces year round, Tivoli east of Rome is the place. Near Hadrians Villa which is probably too hot and dry in July, but the Tivoli Gardens will be so refreshing.

I actually don't much care for Florence, certainly not when the bowl it is in concentrates the summer heat so after Bellagio - 5 nights, 4 days is a whole lot of Bellagio even counting jetlag recovery - I would split the remaining time between Venice and Rome.

I agree that leaving free time to wander can end up being some of the best parts of trips!

Thank you Gerri for the list of gardens!

I know that gardens are different everywhere and that’s part of the fun to me, also that depending on weather and season some things may not look as great.

Nigel we are planning to see Tivoli gardens which I’m so excited about. My daughter was doing a study abroad in Rome (right at the very, very beginning of Covid) and one of the things she is most excited about is going to those gardens to see what they look like in summer since she saw them in January.

Thank you everyone so much for your input!! We are so excited to go on this trip and crossing everything that the pandemic won’t cause us to have to postpone again.