You're only in Y10, you should wait until you have more experience with your subjects before deciding, but I'll offer my advice anyway. If you are good at Maths then further maths is definitely an advantage, I regret not taking it. As far as I'm aware the only thing computer science requires is A-level Maths. Medicine requires Chemistry and most Biology. Thus I would recommend Maths, Further Maths, Biology and Chemistry as the best options for the degree choices you brought up. You would have a strong application for any of the three. It is also not irregular for someone doing further maths at AS to pursue five AS levels, if you feel you are up to this (in a years time) then you could take an essay based subject like English Lit or History to enhance your application for Law and also make you stand out relative to other applicants for Medicine.
As I've said you still have a year and a lot can change. But if you're good at maths and have the opportunity to do further maths then do it, it was a big regret of mine and it is advantageous for pretty much all quantitative degrees at top universities.