While maths and physics should be adequate to get you into most universities, by taking further maths and attaining the required grades for all three subjects, you may make your application stronger.
Further maths may be useful for applying to Oxbridge, however the third subject should be something that you enjoy and are motivated to do well in!
Remember, most sixth-forms/colleges allow you to change your A-Level options up to two-weeks into the term, so focus on picking a subject that you enjoy, otherwise it will be HELL for the next two years