1.) There are people who are admitted into each medical school after three years. However, these people are generally quite rare; you really need to be top-flight in your academics, as well as your extra-curricular activities and your interview. For example, in my class at UBC, out of 120 students, only about 8-9 got in after three years.
2.) This varies from school to school. Usually it's valid for around five years or so, but you should really check with each individual school. In most cases, the "typical" applicant who writes the MCAT during undergrad, and then applies to medical school each year until he/she is accepted won't run into problems with MCAT's expiring. This is more a concern with the older students who wrote the MCAT during undergrad, and then went on to finish graduate degrees, or work for a few years before applying. In your case, this will probably not be a problem.
3.) I think so. While your chances of acceptance will almost be certainly lower in a "foreign" province, it'll give you some additional familiarity with the application process. If you receive interviews at these places, that is wonderful practice. With each interview, your skills at answering tough questions under pressure get better and better. Finally, the application process is peanuts compared to the cost of not getting in for one additional year. If you get in a year early, you also start working as a doctor a year early. The average doctor makes a heck of a lot more in one year than you could possibly hope to spend in application fees. In my opinion, applying in third year makes it more likely you'll be in medical school by the end of fourth year.