I’m someone who only ever did humanities but whose kids performed really well in maths and physics A levels (no humanities), so these are my observations!
If you’re that way inclined (but only if you’re that way inclined) maths + FM + physics A levels are hugely mutually reinforcing. Strong maths knowledge makes physics easier, strong knowledge of things like mechanics (in physics) makes FM modules easier, and for the whole two years all of the curricula are locking together and strengthening all the relevant competencies. One of my kids said one of his physics A level papers would have been near-impossible to score highly on had he not had FM knowledge to help him.
I’d have thrown myself out the window if I had to sit those A levels, but if your brain works that way it’s a cinch. Much as I’m very proud of them, I suspect my kids didn’t have to work as hard as someone getting similar grades in, say, chemistry + English lit + German, because they are a) just fundamentally good at maths and b) had done nothing but focus on maths topics for two full years.
The other thing that might be helpful, if you’re at the stage of choosing A levels, is to think about where your preferences lie between manipulating big chunks of memorised material in the exam hall, versus the psychic pressure of essentially skills-based exam performance. Performing well in Maths A levels isn’t about memory; it’s about having strong skills and being able to identify the right tool for the job. Exam-setters often disguise the question slightly so that you have to think hard in the moment about which technique to use - there’s a lot of mild trickery in maths exams. My kids used to like being able to answer unfamiliar questions by working from first principles in the exam room. I think you have to enjoy the high-pressure puzzle of it, whereas I hate puzzles and really liked the relative predictability of essay-based subjects. (Not saying essay subjects are easier! Just that there’s less outright puzzling.) Lots of mathsy students comparatively dislike physics A level just because there’s quite a lot more memorising involved and you can’t always just work it out in the room.