if you're looking to go to university: maths/fm/physics is probably the most common combination of A-levels among those who want to do a maths degree at a very good university. Arguably it is less work than 3 unrelated A-levels because becoming confident with FM will help you become confident with normal maths.
The only downside is that it puts you on quite a specific path. (other possible degree choices being engineering, physics, computer science and so on)