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.
I think as long as you do Maths and Further Maths, it doesn't matter! My friend got in with Maths, Further Maths and Geography.
It is best you stick to traditional subjects - for example, doing Media Studies as a third A Level will probably not do you too many favours in terms of an Oxbridge application.
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
Why Oxbridge? Why do you think you want to do a PhD - what sort of maths do you enjoy? What other things are you doing to prepare? UKMT? Reading books and which ones?