I think you should use your best referees during each attempt. Is there a problem with them being used for both Canadian, and US schools? If there isn't, all you need to do is ask your referees to hold onto an electronic copy of the reference letter, and this way you can personalise it as appropriate for each individual medical school. There's no rule stating that you can't use the same three referees repeatedly.
As for your second question, when I applied, UBC only had a requirement that one of the three reference letters be from a university professor, or someone along those lines who could testify as to your academic skills. Therefore, I chose a fourth year professor who taught an interesting Neurobiology course, in addition to a close Doctor friend, and my high school Biology teacher, who I've been very close to.
Under the new restrictions, should you wish to apply here, it looks like might you want to pick out your supervisor, a close associate for your personal qualities, and one from a volunteer co-ordinator. Make sure the volunteer co-ordinator can write a good reference letter; leave him/her your resume for more information. I say this only because co-ordinators are notorious for writing: Mr. So-and-so worked 230 hours at the hospital, and was a valued member of the volunteer team. End stop. Not a useful reference letter.
Also, don't forget to ask your referees well in advance.