Each who has posted has made a good point. Primarily, as mentioned, the regional guides will have information for each region that the general Italy guide will not cover. HOWEVER, the main thing to consider is how much information do you need? Is this your first time to Italy? How long will you be over there/in each destination? I ask this because, if this is your first time over there and you will be at each spot for a few days or so, then I would say that the general Italy guide will be more than enough to get you through the highlights of each region, assuming, like most, that's what you came to see. Since you'll only have time for the highlights, why lug around three times the weight and space of books that are providing extensive information, most of which for sights, places, or things you won't have time to see/visit/do? On the other hand, if you will be over there for over a month, with a couple of weeks in each region, then that may justify carrying around the extra books because you will probably have time to visit the extensive sights mentioned.
One thing to consider is that the Rick Steves regional guides don't necessarily expand the coverage of the regions and mention more places to see and visit, but rather give you more information for each of his sights mentioned in the general Italy guide. In the regional guides, you are more likely to find maps for the museums and illustrations to track a walk around town or something like that. He mentions all of these places in his general guide book, but just doesn't have the space to draw it out for you necessarily or expand in as much detail. So, if you would like this sort of additional information to guide you within each sight, then the regional guides could be worth the added weight. But if you don't need these sights drawn out for you, then the general guide (and the lighter weight) will be just fine.
Have fun traveling!