Two are no-brainers...one from your medicine clerkship director and one from you neuro chair or clerkship director.
The third can be from anyone, but I was told by a Dean to aim for a good letter from a fourth year rotation attending. That way you have a letter that says something beyond/different from all the third year comments in your dean's letter. Most attendings are pretty attuned to you need for letters in the summer...I had a renal consult attending write mine and it was great. I have no future interest in renal--but was engaged in the rotation and that was enough for a good letter.
I used the same letters for my prelim apps, with no changes...
