This is false.
Look at Oxford's courses for CS. An example entry requirements are:
https://www.ox.ac.uk/admissions/undergraduate/courses-listing/computer-scienceEvery CS course from Oxford, including CS with Philosophy, recommends:
Essential: A-level mathematics.
Recommended: A-level Further Mathematics.
A-level Math and Further Math I would say are highly recommended/essential if you're going to study CS at oxbridge - as their website states.
For a third A-level, I want to recommend using Imperial's entry requirements for CS - which I think are very close to oxbridge:
https://www.imperial.ac.uk/study/ug/courses/computing-department/computing-beng/#entry-requirementsEssential: A-level mathematics.
Recommended: Computer Science, Further Math, or Physics.
Useful subjects: Ancient Language, Biology, Chemistry, Economics, Electronics, English Literature, History, Languages, Law, Philosophy, Politics, Psychology.
I would advise your first three A-levels being either essential, or recommended ones. So, Math, Further Math and Phyiscs or CS would be optimal.
I would say having Math + Further Math as the first 2 a-levels are really important, but with your third A-level being physics, CS, or any useful subject, is fine also.