You don't need an A* in GCSE Maths to take, or even to do well in, A-level Maths. However, if you don't enjoy it or struggle to keep on top of the material now, it's debatable whether that will improve in A-level (although on the interest front, I can say at least I found the A-level Maths material a lot more interesting than GCSE Maths, which I found quite tedious and dull). Maths is a subject that requires a strong on-going commitment to it, doing problems every week if not one or two every day, to reinforce your learning. The only way to really learn maths (at least at A-level standard) is to practice it constantly. If motivation might be a struggle with that, that can make it harder to keep up and get a good result.
A-level Geography is a perfectly fine choice otherwise, either for medicine or in the sciences besides. Only one medical school that I'm aware of normally expects 3 STEM subjects (Cambridge), and all the others seem to care very little about which subject you take as your third (previously UCL had language indicating it "welcomed a contrasting subject" although I think this has been removed now). For medicine some aspects of geography may well be relevant (e.g. elements of demography), while in the sciences any aspect could be relevant (e.g. demography, environmental policy and sciences, physical geography and geology, ecology etc).
Whether the lack of GCSE Geography will be an issue is down to your school; if they accept students who haven't done the GCSE, then nominally they shouldn't expect that material, or should be able to provide you with advice and/or suggested reading over the summer. Some schools do require you to take the GCSE, as they structure their teaching around the assumption you have taken it. I don't know to what extent the GCSE material is "carried over" to the A-level, whether it is developed further or simply reframed in a different context though...