carol,
I don't agree that a car is the best way to see the lakes, although it could be beneficial in some circumstances on Lake Garda.
Getting from Milan to Lake Como is very easy by train (about 1 hour to Varenna) and you won't need a car while you're there. Exploring the area is best done by Ferry so the car will be parked and gathering dust (and you'll be paying for parking).
From Milan to Lake Maggiore is also extremely easy by train, again about an hour to Stresa. I doubt that a car would be useful there either, and again you'll be paying for parking. Some of popular sights on Lake Maggiore such as the Borromeo Islands are only accessible by boat, and if you decide to visit Mt. Mottarone you'll be using a Cable Car. While there you could also take a day trip to Locarno (CH) on the scenic Cento Valli Railway, returning in the evening by boat.
From Milan to Lake Garda is again just over an hour (using Peschiera del Garda as the destination). If your destination is Sirmione, using Desenzano station is probably better. If you also want to visit other towns along the lake such as Limone sul Garda or Malcensine, then a car might be useful (but boats are also an option). Depending on the time of year, I believe the narrow roads around the lake can be plugged solid by tourists at times, so it may not be a pleasant driving experience. Hopefully Roberto will spot this Thread as he's very familiar with that area.
If renting a car, note that each driver will require the compulsory International Driver's Permit, you'll have to be vigilant to avoid the dreaded ZTL areas (hefty fines) and will have to pay for compulsory CDW for some portion of the rental (I can't remember which part is compulsory).
All of the areas you're planning to visit are reasonably well served by public transit, so I don't believe a rental car is necessary.