From personal experience, I can say that you don't have to have a letter from every PI you worked with. I'd say if you have 2-3 solid letters from other PIs, then it's ok to exclude one, especially if it was a relatively brief research experience. Some schools may ask you to explain why you did not include an endorsement from a particular lab -- you could indicate that you did not have much personal interaction with the PI and therefore did not ask for a letter from him/her.
It's fine. There is no reason to have a letter from every PI, particularly if you have other research letters. I don't think it's a red flag, particularly if it is only a summer program.
I doubt that the letter from the post-doc is needed to try to make up for it. I would use this letter only if you need it.
I think most programs require just two. If anyone asks, just say something like you didn't get to know the professor that well or something (they probably won't ask, I would guess.)
So, you're fine if you have the other two letters.
I think most programs require just two. If anyone asks, just say something like you didn't get to know the professor that well or something (they probably won't ask, I would guess.)
So, you're fine if you have the other two letters.