For the schools you’re interested in:
U of A: https://www.ualberta.ca/admissions/international/admission/admission-requirements/competitive-requirements
UBC: https://www.ubyssey.ca/blog/ubc-admission-stats-2016-averages-acceptance-rates-and-also-baileys-legacy/
McMaster: http://future.mcmaster.ca/2018/05/01/sept-2018-admission-averages/
Here’s the coarser data from the Maclean’s university rankings: https://www.macleans.ca/education/canadian-universities-minimum-entering-grades-by-faculty/ You should double check with the university’s website for the schools you’re interested in applying to, but this gives you a general ranking of selectivity.
For all the schools, keeping your GPA >2% above the ‘competitive minimum’ (or the previous year’s minimum average) will usually do the trick. UBC has ‘broad-based admissions’ to consider candidates with borderline GPAs. From what I’ve heard, the boost effect from essays is pretty small. I don’t know for sure, but I’m curious about it and might volunteer for them this cycle to find out more. Remember that your admission average is only calculated based on the courses they’re specifically asking for, and this depends on the degree program you’re applying for.
Funny thing - unlike almost everywhere else, at UBC, Science has always been the most competitive to get into (and stay in), more so than Sauder or Engineering (applied science). Only this year it seems Applied Science is catching up. There’s a degree of snobbery with the science students as a result (definitely was guilty of this myself).
I have a soft spot for U of A because it’s pretty easy to get admitted, and it’s a high quality school. I’m from Western Canada, and out here U of A is very well-regarded by employers. I knew a lot of high school peers who weren’t the best students back then, but got into U of A and really bloomed and got to discover their talents and mature through the college experience.
At UBC and U of A, you declare your major the summer after 1st year or later (I don’t know about McMaster), and it’s a competitive process. At UBC, biochem was pretty easy to declare (need a B+/A- average), but it requires a more rigorous chemistry sequence through year 2-3 (3 semesters of organic and physical chemistry each instead of 1 each). Microbio is more competitive. Physiology, pharmacology and med lab sci were the most difficult.