Generally, this means that you should arrange for two letters from professors in the sciences and one from a non-science professor. It may be appropriate in some cases to ask an employer or someone else to write a letter for you if he or she is in the position to judge the quality of your work. Every professor from whom you take a class is a potential recommender for you. Don't just go to class - study hard, get good grades, and get to know your professors. Develop relationships with those at the University of Michigan whose letters will mean something to medical school admissions committees.
Medical schools like to see a strong, lengthy relation.
Those who write you a letter of recommendation should speak with authority and experience about why you would make a successful medical student and an excellent physician. Also, ask your recommender to write the letter while he or she is still freshly familiar with your work. It is difficult for someone to write a good letter if they have not had contact with you for two or three years.