No, you don't need three years of relevant work experience for Waterloo, though it certainly helps. On the website, https://uwaterloo.ca/public-health-and-health-systems/future-graduate-students/professional-programs/master-public-health/master-public-health-applying, it only indicated one. However, I was accepted on probationary status this year (last year, I was wait listed, then rejected) because my grades weren't so good.
Basically, for probationary status, I have to end up with a 75% average on two courses - one in the fall and one in the winter. I had a 72% average but over three years of relevant work experience. It certainly helps if you do have the grades AND the work experience. What was also different this time around was that I submitted two work references instead of one work and one academic. My grades weren't great and I graduated in 2009. I took the risk and submitted two work references, since I know my academic reference will be too general. Waterloo was also the only school that I applied to.
Relevant is also highly subjective. From reading the posts on here, the majority of applicants have clinical practice or research experience. I don't have any of that. I worked in a community health centre for a year as an office manager, developing policies and working on budgets for new programs. It was more administrative in nature but had a high impact that affected the catchment area it served. I would say the selling point is my current work experience at a regulatory college. I coordinate assessments for a specific type of regulated health professionals, worked with Public Health Ontario to develop infection control policies, worked with Cancer Care Ontario to develop standards for clinics, etc. I've been in this position for a year and a half. I emphasized how much an MPH would help in my current position and how it will help get me where I want to go. I described that the MPH will give me a unique perspective, as most of my colleagues would be coming from different backgrounds.
So, even with not so great marks, still apply! Just find ways to make it work. You know what your strengths are, so do play it up.