A professional reference letter for pure math graduate school I'm applying for master's degree in mathematics (pure math) at Canadian universities.

I already found two professors to write academic letters for me, but it's hard to find the third one who can write a strong letter.

All schools mentioned on their websites that they require 3 letters with at least two academical references which means the third one could be professional.

I've been working as a math tutor at a community college  in Toronto for past 3 co-op terms. She has supervised me for 2 co-op terms and knows me very well (even more than my professors).

Her position is Learning strategists - math specialist which means her job is to support students' learning and deal with their academic concerns. Before the current job, she earned her master's degree in mathematics and statistics and worked as a lecturer/teaching assistant at a university for 5-7 years and was an instructor at the current college for few years. I don't want to reveal too much about her, but at least I can say she went to/worked at top Canadian universities.

I have built good relationship with her as a co-op student &amp; supervisor and I have some impressive achievements in a college tutor level. So I believe she can write a strong letter for me.

My concern is she doesn't hold PhD. Would committee take her letter seriously? Do you recommend to get a letter from her rather than from a professor who barely knows me?
That is of course always hard to answer, as we don't know you, we don't know what would be written in the letters, we don't know about the person reading your letters. Thus, the answer below is just my personal opinion, it might or might not align with the opinion of the persons in charge of admitting you:

I would go with the letter from your job. Your job is math related, the person writing the letter is working with math daily and she knows you better than the third professor. Thus, you would get a personalized letter instead of the generic "Andrew is a good student and passed my class with an A" that admission boards surely read way too often. Furthermore, it shows that you really enjoy math and not just study it to get a degree. With the right wording, this could show you as a really motivated mathematician; and every professor knows that to do a master's or maybe even a PhD in pure math you need tons of (self)motivation. 

I wouldn't worry too much about the fact that she doesn't hold a PhD. On the contrary, she has been allowed to work and lecture at top universities without having one, that tells way more about her than a piece of paper saying that she wrote a thesis at some point in her life.
Oh, and if she already worked at top universities, maybe she also worked at the one you are applying to and has connections there? That would put her letter even above the two from your professors I'd guess.