Most medical schools require 90 credit hours (roughly the equivalent of 3 academic years) of college credit earned in the US or Canada. I have seen applicants with an undergraduate degree from abroad come to the US and do a PhD thus earning 90 credit hours in the US before applying to medical school. You might be able to do the same with a post-bac (60 credits taken over 2 years) plus a MS (30 credits taken over 1 year).
Typically, medical school in the US is 8 years: 4 years of undergrad in the US/Canada followed by 4 years of medical school. There are some programs that take high school seniors and push them out in 7 years (3 years college in the US, 4 years med school) and there have been 6 year programs (not sure if any still exist).
Bottom line: if you want to be a doctor in the US, you need to study in the US for not less than 7 years.
It might be worth it to buy access to the MSAR (online resource) produced by AAMC. MSAR is the abbreviation for "Medical School Admission Requirements" and it provides info on all the US and Canadian schools.