I'm a soon-to-be 3rd year and I know only ONE student - premed or otherwise - graduating in 3 years. She just happens to be the most mature person I know, who has held down a job for the past, oh, 5 or 6 years in addition to volunteering ~10 hours a week. Even she is scared of the prospect a little. College is a time to mature. I'm sure you're mature enough for college, but most of your medical school classmates will be around 25 years old (avg entering age) and you will be a BABY compared to them, in terms of age, life experience, and probably knowledge. If your parents are so wealthy that they can pay for college without batting an eye, take the 4 years and take some electives. I don't know where this "lower" and "upper" division coursework comes from. For a bio major here, we have Gen Chem (year), OChem (year), Physics (year), 5 quarters of bio + 5 more bio classes, plus biochem, plus a year of calculus. We are on the quarter system, limited to 4 classes a quarter, for a max of 12 classes a year. We also have extensive core requirements which total to 12 more classes (and that's only if you take Gen Chem and Bio to fulfill the remaining 4 - if you're not a science major, you need to take 16 core requirements).
In 3 years, with AP credits and placement tests, the girl I know will graduate fine. But she is missing an entire year's worth of classes and experiences just to be a doctor ONE YEAR EARLIER. It's not worth it, if you ask me, or any of the other people in this thread. Try being financially independent and in college at the same time. It's quite a bit harder to manage than you have it right now. Not everyone gets to volunteer 200+ hours at Daddy's office, because Daddy doesn't always have a lucrative practice and some people actually have to work - yes, for money - to get through college. So maybe you should reconsider your world view that 4 years is "slacking." I'm pretty sure the reality is that you have it too easy to know why anyone would take 4 years to - gasp! - graduate.