Hi, so I have had 3 research experiences throughout undergrad--one was a summer program after freshman year, one was a year long research project, and the third has been 2 years (MME) and I plan on continuing the third experience in my gap. I don't have the best relationship with the summer program PI so I really would rather not ask them, and the 2nd PI often takes > a month to respond to any communication--is it a red flag if I would only have a letter from the 3rd PI? I'm definitely not applying MD/PhD if that makes a difference. Thank you.