Really don't understand why the need to do more than 3 A levels. There is absolutely no need to.
In the case of students I know personally, the decision to continue 4 (as I say, both local schools that my DCs go to start with 4, so that's the norm) has not been based on a purely functional decision about what is 'needed'.
For some, it has been out of a genuine love for the subjects, a burning academic drive, a wish to retain a broad base (e.g. a language alongside the 'functional' 3 sciences) from students who have found the narrowing inherent to A-levels irksomely restrictive.
For others, who have chosen 3 + Further Maths, it is because A-level Maths is insufficiently challenging for them to enjoy (and it's not an easy subject to 'read around', hence choosing the further Maths route) / they have needed further Maths as a basis to study Maths at the best universities and they have wanted to retain 2 other subjects alongside Maths, through intellectual curiousity.
Again, from the examples i have known, many have maintained e.g. county level music, weekend jobs, involvement in school / sixth form activities such as debates, shows, competitions alongside their study - it's been a route for polymaths, not those who have no interests outside their narrow A-level studies.
IB is not available in state schools locally - many of those choosing to maintain 4 broad-based A-levels might have taken that route if available.