Hey, you should add more clarity as its hard to understand what you are trying to say. So you have already started a levels with those 3 subjects and want to change them or what? A level maths is doable even with starting a little late, but physics is complex and in fact i started my study in November 2015 for physics since they did not offer it in September and they gave us the opportunity to switch to physics in another campus. I got a U grade for physics and loads of my holidays involved going to extra revision lessons with teacher to catch up which messed up my study time for other subjects (ended up failing year with 3 U grades). If you are choosing to change A levels close to December, that is really not OK because i remember trying to enrol into physics on October 30th 2016 to retake and the admissions officer said they had done all the practicals already in the first 6 weeks of college, now im doing BTEC extended diploma Business which i started on november 2016. Just wanted to say I can see myself in your situation just last year and it did not go down well i failed both AS years and ended up in BTEC. I recommend you do BTEC extended business or something if you like money since they don`t require any prior knowledge of business for entry. You are unsure 3 months into AS level just like i was and i really recommend you do not switch subjects because I fell prey to that and failed.