For grad school you should probably not go into physics (although don't completely disregard it), with engineering its a matter of taking complementary classes and tutoring to catch up from a foundation in physics and with computer science it depends on how well you can teach yourself an...
Engineering is great but Computer Science/Software Engineering is great too. With your interest in helping others in a medical field way you can either apply your engineering or programming skillsets to medical technologies, both have well respected & high paying senior positions as career...