Is the sixth form you are applying to offering A levels and AS levels separately? Because at my Sixth Form, only AS level Maths and Further Maths were offered. Everyone taking the full A level took the AS, so regardless of if and when you dropped out, your learning wouldn’t be a waste. However, for all the other subjects, no one could take AS. Because of my scores in the AS exams, some of the universities I applied to improved their offer, so if you can take a fourth subject as an AS, I would look to see if any potential universities you may go to take AS scores into account when writing offers.
You do not need to take the AS of a subject to take the A level. Most people don’t take the AS exams.
Cambridge and Oxford want at least 3 full A levels.