I'm applying for a PhD in CS. I have been worrying about what my third LoR since I don't have an ideal candidate to seal my other two letters. Right now my first letter would be from a professor from the physics department I did research with for two years, and my second one would be a professor from an REU I did this year, however, I'm not sure who I should go with for the third LoR. Since I don't have anyone from my faculty yet I feel like I should get someone who is faculty at my department.
My first option is a professor I have a very good relationship with, I took a year long class with her (software engineering) and then I went to help her as a TA the next year. I feel like she can attest to my leadership skills, as well as speak very well about how well I work. The only problem I can find with her is that she doesn't have a PhD, she's an MS.
My second option is a professor that specializes in something close to what I want to do, but was largely against students doing a PhD fresh out of a bachelor's. She likes me a lot though, so I may be able to convince her?
My third option is another professor that also does something similar to what I'm interested, but I feel like he wouldn't give me a letter of recommendation as strong as the other ones, however, he would still be willing to help.