Doing more A levels than expected does not increase your chances at Oxbridge. You may do 7 A levels and still be rejected. You need 3 A levels, good predicted grades, good personal statement etc and to impress at interview. If you can handle it and would enjoy it, then by all means, take 5 but it puts you in no better position than students who excel in each area and have only done 3 a levels. Of course it looks great to get 5A*s, but in the end, you may feel too pressurised by the workload and end up getting less than that. Best to stick to 3 and do well in all 3 than to do average in 5 subjects.