Can I ask your reasons for wanting to go to Cambridge to do medicine?
Also, I know two people who did 5 A Levels, same ones as your are suggesting: one got 4A* 1A, the other 3A*s 2As. They also both did AEA and received merits. They're both at Galway doing medicine. It is definitely not necessary, but it is possible. They devoted so much time to their studying, though, and were naturally very good at maths anyway.
If you're set on doing those. Drop either FMaths or Physics at AS. Doing 5 is extremely tough unless you are very devoted and have very good organisation and time management.