For most programs, you need 3 letters total. One from a pathologist is a minimum. Two might be nice. Three, all from pathology, may be overkill.
That being said, you want strong letters...but I don't think they are absolutely required for securing an interview. I've read quite a handful of generic, what seems to be canned letters that are only like 4 paragraphs long. Half of the letter describes the rotation and the requirements. Then it goes into saying how the student exceeded those expectations and then of course there are a few sprinkles of descriptors of the applicant's personal qualities. But nothing in these letters blow your socks off. But I guess these letters are good enough to get people interviews. And then at that point, what matters is not the letters but how one performs on the interviews.
Moral: Just get your letters. If one is awesome, more power to ya.