Like everyone else said, this offers less time for EC's, research, shadowing, etc... But if you can get all those under your belt in 3 years and still come out with a good GPA and MCAT, then I think ADCOMM's might see you as a very motivated applicant who can handle an intense science curriculum (very good attributes to have when applying to med school).
However, I would never do this. I loved my college experience and I wish I had another 4 years. Also, if you don't handle the curriculum well (i.e. lots of B's) and don't devote the time to preparing for the MCAT, you'll have a hard time getting in. You have to make the cut on paper first and foremost (~3.5 or above and ~30 or above on the MCAT)!!!!!!!!!!!!! It's obviously possible to get in with stats less than those, and avg. GPA and MCAT's vary from school to school, but this is what most people quote as the golden standard for getting in. I stress making the cut on paper b/c I did not devote the time I should have to school work and wound up with a lot of B's. My final GPA was a 3.36 and it has taken me 3 app cycles to get in, something you want to try and avoid. It also takes a lot of time to prepare for interviews, something else I underestimated.
Point is, unless you are a superstar, I don't think you can be confident about your chances of success in a 3 yr program. Take the extra year and enjoy yourself, but still work hard. You'll save yourself a lot of grief.