For what subject? For Maths/Physics/Engineering/CS for example, usually those three are perfectly acceptable without any other subjects - up to and including Oxbridge (certain colleges may be exceptions though).
For other subjects though, it may or may not be acceptable. For example, LSE only considers FM as a fourth subject for some of it's courses (for example it's single honours Economics degree programme - however the joint honours courses in it's maths department are happy to consider them as part of three subject combinations). Other courses/universities may have similar policies (medicine quite commonly only considers FM as a 4th subject, although it's rarely relevant as most are taking Chemistry and another lab science anyway), so you will need to check individual requirements to ensure it's acceptable.
However presuming your are primarily interested in mathematical oriented courses given your choices (and potentially physical science oriented options for that matter) it's should be generally acceptable for any that's likely to interest you. If your school allows you, it may be worth considering starting with a fourth subject and then dropping it at the end of year 12 (which is around the time you need to start thinking about what you want to apply to for uni anyway, and will probably have a better idea of what is or isn't necessary from your subject choices at that time).
So, for subjects related to those it's normally acceptable, for others it varies but often is fine. That said FM isn't necessarily "more work"; most mathematically able students on here seem to comment that Maths and FM are qualitatively less work than any other combination of two of their A-levels (if they're doing four). Of course if maths isn't such a strong point then it may well be more work for you specifically, although given you've planned to take this combination I doubt that is the case.