My understanding is that while you can technically attend medical school after three years of college, almost no school admits someone who doesn't have or won't have a bachelor's degree by the time they matriculate. Try flipping through the MSAR, as I believe they list the percentage of students with bachelor's degrees (or perhaps they only list those with master's).
Regardless, I think you'll have a much better chance at getting into medical school with a BA or BS. Only about 40% of pre-meds who apply are accepted, so medical schools can be very picky. Unless you have a really stellar GPA and MCAT, as well as EC's, I would imagine not having a degree would really hurt you. Of course, this is just one person's opinion, so if you have recieved a lot of advice from advisors and adcoms that you'll be fine, then disregard what I said!