3 days early in venice

Go up to the Dolomites for a few nights! Some of the most beautiful scenery in Europe.

Arriving a few days early for a tour is a really good idea in general, prices aside. I suggest staying in Venice proper, not on the mainland. Not sure you'd want to do a day trip, with that short a stay, but Padova is only 1/2 hour by train, and visiting the islands of the Venetian lagoon by vaporetto would be fun.

Slovenia? You can go directly from the Venice airport to Ljubljana (look into DRD or GoOpti, maybe even FlixBus). Ljubljana is one of my favorite cities in Europe. If you are going after June, I understand that there will be direct trains between Ljubljana and Trieste again after that, so you could train back from Ljubljana to Venice via Trieste.

I'm guessing that you're on the Best of Venice. . . tour. My first thought is that if you haven't booked flights yet, look at flying into Milan - you might get better prices. From Milan, you could go straight to Venice the day before the tour. Or you could spend a couple nights on Lake Como and/or in Verona. Both are easy to get to by train from Milan. If you are flying into Venice, then Verona would be my recommendation.

I suggest staying closer to Venice for 2 nights and then going to Venice the day before your tour begins to see some of the sights not included in your Venice itinerary. Good options for the first 2 nights are Padua (direct bus from airport or RV train from Mestre), Vicenza (regional train from Mestre) or Verona (Freccia or regional train from Mestre). There are Freccia trains from Mestre to Padova, but they're no faster and much more expensive than the RV (regional voloce) trains. If timing works, the RV train from Mestre to Verona takes only 15 minutes longer at one-third the cost, but they don't depart all that frequently.

Thanks everyone....what is distance and easiest way to get to dolcimites without renting a car from Venice airport

From Venice airport you would take a bus to Venice Mestre train station, then have a 3-4 hour train ride to Bolzano and finally would take a bus to your destination town in the Dolomites. The trip will likely take a minimum of 5-6 hours.

I am guessing you are taking the Best of Venice, Florence and Rome tour in which case you spend 3 nights in Venice which gives you 2 days with your tour. Arriving 3 days prior to the tour would give you alot more time to explore Venice on your own and wandering around getting "lost" in Venice is fun! Or as Frank recommended take the train to Padova, Vicenza or Verona. Padua (Scrovegni Chapel, make reservations) and Verona are lovely. Haven't been to Vicenza. I am sure there are people that do all 3 in one day. Yes, the dolomites are beautiful and we, too, loved Ljubljana but particularly if this is a first visit to Venice I would just spend the time here. Have a great trip!

To give better recommendations, we need to know which tour you are taking. How many nights and days does it have in Venice? Where else does it go?

From the Venice airport, there are direct buses to Venice Mestre station; from the station, you can get trains to anywhere in Italy.