Buongiorno,
Italy and Tuscany should be on everyone's bucket list
Three days in Rome sounds great - and if you leave an extra day there before you fly out, it will give you a chance to take in that lovely city one more time or see something you just didn't have time to the first time around.
As for Venice, many people do it as a day trip from Florence. There are quite a few easy trains to catch that will get you there and back in one day with time to see it all. However, if you want, then do a night there to get the full experience - be sure you understand where the hotel is, because Mestre requires you take a train to see the city. I have stayed there several times, once I opted on the island in front of San Marco (so not on the main touristy area) and it was lovely but you do have to travel back and forth.
I would suggest that you come to Florence - at least two days to see the main attractions, beauty and culture. Then train it to Venice, stay the night.
When you come back to Florence, you could choose to stay near the city so you take advantage of some of the great organized tours that will show you around the region, small towns and wine tours while really absorbing the beauty of the landscape.
Have a look at these places:
https://www.florenceaccommodation.com/villa-lysis.html
https://www.florenceaccommodation.co...-lilliano.html
https://www.florenceaccommodation.com/torre-a-cona.html
https://www.florenceaccommodation.co...afaggiolo.html
I also suggest you have a look at this article where we highlight some of our favorite organized tours to see the best of Tuscany in spring - if you are not going to have a car while you are here then these are great alternative options for getting the some quality time with the landscape of Tuscany:
https://www.discovertuscany.com/tusc...in-spring.html
Hope this gives you a starting point,
Buon Viaggio,
Donna Denise
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