here's what they say:
Letters of Recommendation: Letters from your college Pre-medical Advisory Committee or a college letter service are preferred. You may disregard the waiver forms if a committee or letter service is available. If your college does not have such a committee, appropriate letter writers include science/non-science instructors, employers, community service agency directors/coordinators, and faculty members supervising undergraduate/graduate research experience. No more than THREE letters of recommendation will be accepted. Please indicate in this application which letters we can expect.