I worked on three research projects over my undergrad, and two of them are with professors and one of them is assisting a phd student (so getting a letter here is almost impossible). Only one out of two of my research mentors is available and willing to write me a letter. The other mentor maybe I asked too late maybe he didn't like my work. Would if look bad if I write about three research projects in my SOP and only have one letter from research mentors? (FYI, the other two letters I am asking my professors who I took classes with.) What is the best strategy here, not heavily mention the other two projects? Omit one project from my resume?
Also, for masters programs that are heavy on academic and research, would it look unprofessional to get a letter from an internship supervisor?