Assuming you are renting, be aware of the exorbitant fees you will incur by renting in one country and dropping off in another country, likely hundreds of Euros.
Make sure you know the rules of the road for each of the three countries, including traffic enforcement stops, demands for immediate payment on speeding tickets (it's happened to many of us), and ZTL restrictions in many Italian towns and cities.
You will need a good map (Michelin makes the best) and will need to be careful concerning parking.
I drove a 19 foot long motor home from Paris down through Provence and into Italy. It was a lot of fun, but I returned it to Paris on my way back. Gas/diesel costs a fortune, but it is just the price of doing business.