If you want to do medicine, I'd strongly recommend doing Bio and Chem, the third doesn't really matter too much, Maths is a strong subject (in general, not specifically for medicine), but most Unis don't really care what the third is in, just that it's there.
have you considered doing Psychology as a 4th A Level? It'll mean extra work (obviously :P) but it'll help you to stand out from other candidates, and if it interests you, and you enjoy it, that shouldn't really be a problem for you