I think I'd agree with Cristo. Mine is a "non-academic", and "did my A-levels in the eighties" opinion, so things may have changed. I actually chose the same, Double Maths and Physics, and that was certainly enough to be working on. I'm sure my abilities are no way near in your league, so maybe a fourth A-level would be possible for you. I would suggest Computer Studies, as that may not tax you too much except for actually doing the work. I would only do it though, if it was actually going to be useful for you in the future other than having an extra A level to your name, which I do not know. If it wasn't, I'd stick to doing three.