Italian rail fares are comparatively low, so including Italy in a multi-country rail pass is almost never a good idea.
It's quite possible that you will save no money at all with a Eurail pass. If you take any TGV trains in France, you will need to make and pay for seat reservations. Furthermore, there is a quota on seat reservations for rail-pass holders, so it is possible that you would not be able to get a reservation for the train of your choice even though there were seats still available for purchasers of individual tickets.
When is your trip? How firm are your travel plans? If you are ready to buy tickets now, you can look on trainline.eu or SNCF.com to see what you would pay if you bought tickets today for the trains you expect to use in France. Note that the cheapest tickets for TGVs (if they are still available for your dates) are non-changeable and non-refundable, so you must be very sure of your itinerary and timing before making a purchase. But early purchase yields really substantial savings.
For Swiss fares, use trainline or the SBB website. Swiss rail fares are quite costly mile-for-mile, so you might benefit from purchase of one of the Swiss cards or passes even if a two- or three-country Eurail pass would not pay off. A number of people on this forum have found the half-fare card useful.
For Italian fares, it's probably best to use trainline since there are two companies running trains between the major cities, and you can see all of those options on the trainline website.
To see what works for you, you need to take a look at the fares you would pay without a pass.
