My LOR situation.. advice?

[sje]

I have two professors who will write me strong LORs--one from a professor whom I've had for one course and am doing research with and one from a professor whom I've had for three classes and has established a rather positive rapport. The situation is that for some of the programs that I'm applying to, 3 letters are required.

I have a few professors in mind that I could ask to write a third a letter, but I know they won't be that outstanding because I just don't know them quite as well as my other two professors. One of them is someone whom I've had for 2 classes, but one class was a large (150+) intro class and one was an online 200 level course due to the professor being away doing field work during the semester. She knows me and knows that I'm a good student with strong interests, but I think that's about the extent of it. The other one I have in mind is a professor whom I've had for two classes (both 200 level); however, she's a biocultural anthropologist and I get the feeling that she's more into cultural anthro than biological. Her classes were fine and I did well, but they weren't directly related to my areas of interest.

So, what do you all think? Which one do you think I should approach with the prospect of writing me that third letter? Just as a side note, there's one other professor whom I could ask to write me a letter, but he's from a different school and department (mathematics). I know he would write me one, but I'm not sure if it would be detrimental coming from such a different place and department and that I had classes with him >2 years ago. I kind of just have him in the back of my head at this point, but its an option none-the-less.