Letters from a different department than my grad programs

[Morgan R]

I am having trouble deciding on who to ask for my third LOR. My undergraduate degree is in psychology, however, I am applying to graduate programs in statistics. My first two LORs are both from psychology professors. I conducted research with them, I was an RA for both of them, and they both mentored me throughout my senior research project. I am very close to them and I am hoping to receive strong letters.

For my third LOR I have two options:

A third psychology professor, whom I was a TA for and took two classes with. We are fairly close and I am pretty sure she would be willing to write a letter for me.

Or, a statistics professor, whom I took a class with and met with on multiple occasions. I believe that she would also be willing to write me a letter. However, she either is a graduate student herself or recently graduated with her Master's (no PhD). I am not sure how important it is for an LOR to come from a professor with a doctorate. And I don't think a letter from her would be as strong as one from my psychology professor simply because I did not work with her as much.

My main question is, how important is it to have at least one LOR from a professor who works in the field in which you are applying to? Would it negatively affect me if all three of my LORs came from psychology professors when I am applying to all statistics programs?

Any feedback is greatly appreciated!!