Right now I have 2 great letters of recommendation from my research advisers. I am having a little bit of trouble deciding about the third letter.
One option is that there is a third professor in my group, however I rarely talk to him. Also I am not too sure how wise it would be to have all three letters coming from the professors in the same research group. I asked a nuclear engineering professor who I know well to write me a letter, but he is concerned that his letter will not be as strong since he is not a physics prof. He said that he would gladly write me one if needed. Another option is that the chair of undergrad studies will write a letter talking about the strength of the department's program and the strength of your academic progress. I am not too sure how great it would be, since I attend a well known, large state research university.
I am strongly leaning towards using the NE professor, since I have taken a few classes with him and have talked to him outside of class. I think it would be hard to make a similar contact this quarter in physics. I am a little worried though, because I didn't think that him not being in physics would be an issue (basically thought that anyone in a STEM field would be OK). Since I have a mediocre GPA (3.5), I am trying to strengthen the other parts of my application as much as possible. I will be applying mainly to Applied Physics programs.