The third A-level doesnt matter, the mains are chemistry and biology for medicine do what u are going to be better at trust me you'll thank yourself later i know people that done biology chemistry and PE and got into medicine. Do something to make your life easier not harder, ignore your school at the end of the day its your future. i personally picked biology, chemistry and psychology and my the other people in my year that want to do medicine they do maths instead of psychology and they are struggling a lot. Its different for everyone though.