Hello, thank you for your question.
If you considering doing a science degree in the future, you will need at least 1 Science A Level (or equivalent), out of Chemistry/ Biology/ Physics; sometimes two depending on the degree title and the university. If you are more familiar with Chemistry, that would probably be a better option for you; plus as a rule of thumb, for most science degrees Chemistry would suffice, whereas Physics may not. However, ultimately your choice should depend on what degree/career you would like to go into. If you are unsure, I would choose facilitating subjects such as Chemistry that allow you to keep your future study options, as open as possible.
On the other hand, if you are considering a non-science degree, then usually any 3 subjects at A level would meet the entry requirements. However, of course you should check this before making your decisions. Saying this, I would still encourage you to choose the most relevant subjects at A Level for a given degree subject, since this will make the degree course that much easier for you.
Myself I have taken 4 A Levels. Overall, it has not impaired my grades; although it did require me devote a lot more time towards my studies. Thus, I would only recommend doing 4 subjects, if you think this would benefit you in the future, in terms of your academic knowledge/ experience, and provided that you can commit to more study hours. It is worth pointing out that taking 4 A Levels will not put you at any advantage, in terms of securing a place at university, since only 3 A Levels grades will be taken into account.
I hope this helps. Let me know if you have any questions.
Best Wishes,
Kasia (4th Year Medical Neuroscience Student, MSci with a Year Abroad).