I am wondering what 3/4 A levels you recommend if I am aspiring to study Maths at Oxbridge and complete a PHD in the subject
Maths, Further maths and anything else for Oxford. They really don't have a preference for the third A-level. Physics perhaps a good third choice for Cambridge where the first year is a little more applied. Chemistry would have no benefit for either maths course and there is no preference for 3 vs 4 A-levels.