I would say that applying after third year is an advantage. The chances of getting in after third year are slim, (you can see in the UBC statistics I've posted that less than 10% of the class usually gets in after third year) but there are many benefits even if you don't get in. First, you get to visit UBC, and perhaps get a better idea of the learning climate here. Second, you gain the interview experience that will serve you well on your next application to medical school. Third, if you meet other pre-med students, or current med students, you'll have a chance to compare notes and thoughts on the other med schools in Canada. Those other students are often a great source for getting the inside dirt on each school. Fourth, you may get accepted, and have one less year of undergrad to worry about.
Unfortunately, since it sounds like you haven't yet completed the MCAT, and you are currently in third year, the earliest MCAT you can write is April 2000. Applications to UBC medical school for classes starting Sept 2000 stopped on Dec.1, 1999. This means that the earliest UBC medical school class you could enter is the Sept 2001 class, by which time you'll already have your degree. The MCAT always has to be completed before you submit your application to a Canadian medical school.
In order to be accepted as a third year student, you almost invariably have to have a high GPA. My guess is that it should be around a UVic GPA of 8 or so, which translates to an A, or 85%-90% for all your classes. If you have an undergrad degree, of graduate degrees, the marks can go down a bit. UBC doesn't value the MCAT very much, although a high MCAT never hurt anybody, and it would be crucial if you wished to apply to American med schools.
To study for the MCAT, please see the MCAT section I've listed on my web-page.
I recommend setting aside three months to study (I think you should take the
August 2000 MCAT, and not the one in April), and reviewing from your own notes
for understanding, and a good review book for thoroughness and full MCAT coverage.
Finally, I'm absolutely positive that I'm enjoying medical school more than
I enjoyed undergrad, mainly because I really, really enjoy all the classes
I'm taking here. UVic Biology combined with med school pre-reqs does not leave
a very flexible schedule, which forced me to take some bad courses along with
the good ones.