You are going to an area which will be chilly if not cold at that time of the year. If you like to hike the Cinque Terre would be nice at that time of year and you won't be bored with 3 nights. The trails from end to end are about 6 miles and from relatively flat to very hilly. If you're not into hiking two days and one night should suffice. Obviously water things are out of the question. You can also get storms then as well. We were there in late September and one day is was in the 50s with winds in the 35-50 range and two days later 70 and calm. I had never been to Venice and Milan until last year when I went there on a solo trip that also included Florence and Rome. Find a small centrally located hotel on the island (see RS guide book) and don't stay on the mainland. The best time in Venice is early morning until about 10 and later in the day after about 4. In between is when all the tourist come in from their ships or the mainland and the area between St. Mark's Square and the Rialto Bridge are literally packed, even in October. That is a great time to walk around the city away from the crowds. Amazing churches, museums, regular locals, restaurants and more reasonably priced shopping. Be sure to go to Murano and Burano in the lagoon. If you are there for three nights get a 72 hour vaporetto pass. If you go early or late to St. Mark's Basilica and the Doges Palace the lines are shorter. St. Mark's Square at night is magical, as well as a ride late night vaporetto ride down the Grand Canal when it is all lit up. Others can tell you about Lake Como, but from everything I've read about the lakes they are more of a summer destination. You said you'd been to Milan. Did you see DaVinci's Last Supper or climb to the top of their cathedral which is spectacular? I planned my whole two-week solo trip around my LS reservation and it worked out fine.
We loved Provence where we (three of us) rented a farmhouse for a week in St. Remy and drove in a different direction every morning (like spokes on a bicycle wheel). Fascinating places everywhere. My must sees are the Pont du Gard, Palace of the Popes, Cassis, Roman ruins in Arles and Nimes, and the Luberon wine country. If possible go to Gordes on market day. We were there in late September, so October should be absolutely perfect weather in Provence. You didn't say how long you will be there but I'm assuming with the itineraries you listed its about two weeks. All of the above work with that. You can take the train from Milan to Avignon. If less than two weeks I'd consider dropping Italy or Provence and make it more about quality than quantity. We did Normandy and Provence in two weeks and had plenty of time to see everything, taking a train from Paris to Bayeux where we rented a car. From there we took the train to Avignon and rented a car at the TGV station and the train back to Paris. You can also fly into Paris and out of Marseilles or vice versa, but the train back was more economical and got us the right flights back hone. Hope this has been helpful. Have a great trio and if f I can help please let me know.
Jim