Ciao and welcome to our forum!
You definitely want to do train between Florence and Venice and from Florence rent a car to see Tuscany. So you might consider flying into Florence, staying in Florence a few days first, heading to Venice by train and then returning to Florence to just rent the car and set out to see Tuscany..... or flying in to Venice, see Venice first, then train to Florence, stay in Florence, then rent car and head out to explore Tuscany. Maybe this second one is more linear, but all depends on what you find as far as flights and such.
Another alternative is to fly into Pisa airport, rent a car there and see Tuscany first, then drop off car in Florence, spend time in Florence and then head on to Venice. Fly out of Venice, if it isn't that much more expensive.
The Florence airport is Amerigo Vespucci, the code is FLR, the Pisa airport is Galileo Galilei and code PSA. The Venice airport is Marco Polo and the code is VCE.
My suggestion is at least 2 days in Venice, at least 3 days in Florence and dedicate at least 5 days to Tuscany, if not a whole week. It all depends on how much time you have, but if you don't want to be in a rush, I suggest planning for more days rather than less so that travel time between towns doesn't eat into the time you have to see each place.
As for Tuscany, I suggest staying central, like to the south of Florence. That way you can drive out and explore the Chianti wine region, Siena, Pisa and the Val d'Orcia area south of Siena (classic postcard Tuscany). These are the main sights and areas to visit for anyone coming for the first time to Tuscany. I am just giving you a general feel of the area, but if you keep reading the forum and the website, you'll see there are many small towns and sights in the countryside as well where you'll want to spend time (San Gimignano, Volterra, the towns in Chianti, etc). So if you manage more days in Tuscany, the better! Also if you want to spend time at the beach, that's a whole other set of places to consider, and while it's possible to do a day at the beach, it would make more sense to maybe choose 2 separate bases as to spend less time driving so that you can concentrate more time in each area.
As you make plans and they become more concrete, do post your questions back on here and we will try to keep offering more ideas and recommendations! ;-) I am always excited to travel "virtually" through all of your shared planned itineraries on here