As the poster above me said, 3 A Levels is perfectly fine for Medicine. They would SO much rather you did 3 and absolutely aced them than do 4 and get an average mark and not meet your entry requirements. Which one out of Maths or Physics are you better at pick that one. If you're equally good at both, you have to take into account that physics has coursework whereas maths obviously doesn't. If you're not a fan of coursework or it will be set around the same time as coursework for bio and Chem then maybe choose maths