score:5
It's not very clear what you mean with "explore the area between" since you will spend all days in cities. Anyway, since I guess it just means how to move from Milano to Roma, you have possibly two ways:
The train sounds like your best bet. You don't have to worry about driving in a country you don't know (I presume), find a park (expensive in those cities), deal with traffic jam etc. In Roma and Milano you have a somewhat nice metro/tube/underground that can bring you everywhere while in Firenze you can use buses (even tho just walking in the city center will keep you busy enough for two days).
Italy has an extensive net of trains and buses so if you want to visit the countryside you won't have any problem in doing so without a car.
Anyway, I wish to add my unsolicited-subjective-opinion: 3 days in Roma and 2 in Firenze aren't even enough to see all the beauties those cities have to offer. I really think that, considering the time you will spend moving between cities, go to the airport etc, visiting the countryside would really be too much. If you really, really, really want to do so you may want to stay only one day in Milan and save that extra day for visitin around Firenze, which is very beautiful (you may consider to visit San Gimignano or Siena or Monteriggioni or Lucca etc etc).