I do not know of any organised tours, but you could easily do day trips to interesting places. As well as Florence, other possibilities include Padova (30 minutes by train) which has one of the oldest universities in the world, a stunning duomo, and the incomparable Scrovegni Chapel; Verona (90 minutes) which has 2 genuine Roman theatres and a load of nonsense about Romeo & Juliet; and Vicenza (45 minutes) for the architecture of Palladio.
First look carefully to deal with more than geography. Choose a destination that allows you to remain there for two of the three days needed to accomplish these expeditions. If your total travel time consumes more than the time needed to get something substantial from a three day visit then recalculate your options. Spending too much time going to and returning from destinations might be counter productive. Maybe somewhat closer in terms of mileage than Florence because that destination calls for more than the short time your proposed travel ambitions will allow. Verona might be worth your time. Trieste perhaps Florence might be a little far for all the time needed to negotiate the distance and handle a self imposed schedule determined by many things beyond your control and then return in time for making something of such a lengthy voyage. It is one thing to console yourself by persuading yourself that high speed trains will turn things around for dealing with lengthy distances for minimal consequences from short visits. Padua or Mantua might suit you also. Vicenza also. Staying within the neighborhood or vicinity of Venice is a good strategy to follow, Do not automatically rule our Florence but keep in mind that Florence is more than a small handful of things to see and it might call for more than what ever your short time schedule will permit. Choose a destination that does not call for more time on the train than the time needed to do justice to the destination. Maybe Treviso. A return visit to a place that has enough to keep you in place for more than you intend to remain there the first time might help you organize your time better. Verona might do that for you. Vicenza might do that for you also. Do you like to view architectural design. What type of art do you consider as your priorities. Area around Venice has several different types of art on display.
With the pandemic going, it's probably harder to find tours. You can use the internet to check agencies and user blogs. Even if you find a tour though, don't commit to booking anything as you won't know whether the agency will stay afloat.
Specifically though, a 3-day tour that focuses on art might be hard to find. In your case, let's say you want to visit Florence. You can look into joining local tours for the city or even specific guided tours (if available) that focus on an attraction in particular. If you aren't after scholarly correct information, download the Rick Steves app and you can then listen to his audio tours on Florence, Uffizi, Accademia, etc.
I am Sorry, I did not understand that Trailer Gipsy asked about escorted tours from Venice to other cities. Small cities around Venice would be better destinations for Bus Tours than far cities like Florence and Rome.
I think that paying someone to travel on the Venice-Florence train with you wouldn't make sense. I am sure anyone can take a train without help, then you can use the money you saved to pay for a better tour around Florence led by Florentine guides.
Isn't there a river cruise departing from a town close to Venice? I remember I read something about it on Trenitalia's corporate Magazine.