I'm likely going to get 3 A2's and 3 AS's only, for a Physics Degree should I...

..do a further maths AS too? Or no?

Are you going to be studying those AS's and A2s or.. (basically, what situation are you in when it comes to qualifications you already have? I ask because it may be an option to do something like.. take up AS FM in Year 13, which may or may not benefit your application as well as your knowledge.