The classic combination is two science professors, one non-science professor, plus a physician or a clinical or research supervisor who's worked with you directly. Many schools also accept a single committee letter instead, if your undergraduate institution offers that service.
Choose people who genuinely know you well, not simply the most impressive title available to ask. A detailed, specific letter from a teaching assistant who ran your small discussion section usually beats a generic, vague letter from a department chair who barely remembers you. Ask early, and hand each letter writer your CV and personal statement draft so their letter has real material to work with.