Usually you can get into Medicine courses with two hard sciences (usually they prefer Chemistry and Biology), so I think you would be okay dropping Maths for a different subject, however some universities don't really like Sociology A Levels. If you're looking at a uni that wants three hard sciences, as many do, you'd probably be best keeping Maths or swapping it for Physics.
You should start looking at the unis you might want to go to so you can properly decide which ones would be best for you.