A professional reference letter for pure math graduate school
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.