I did all 3 sciences and maths - this is probably the combination that will give you the most work. However, I managed to keep om top of my work as well as having (a bit) of a social life. This combination is doable, but if you aren't a science nerd like me then it will find it harder to keep up with the work load
However, if you work hard, keep on top of homework, do lots of past papers, you should pass A levels in these subjects, The thing is, doing past papers is always a good idea, as you get a feel for what the examiner wants to see when marking papers,
Make sure you remember key formulas, as many will not be given to you in exams, unlike GCSE. (Physics is an exception, you get a formula booklet will most of the formulae you need, plus some you won't use in AS).
Finally, make sure you get a load of stationary! You get through a lot of paper and ink in A level, especially if you do essay-based subjects (I can't offer a great opinion there). Also, you may want things like flashcards to remember the formulas that you are not given, plus some definitions. I think that was what helped me in the exams.