Bride, it is common in medicine and economics, and presumably some other STEM subjects.
Three subjects is not much, and if two are maths, there is not room for much else.
Ironically courses that say that they do not consider double maths, often really like FM when offered as a fourth A level. I suspect some of the "experts" including math teacher husbands are not aware of how very mathematical and data driven some degrees are now.
DS read economics at LSE (graduating in econometrics) and he reckons he could have got away with only two economics courses, with the other 10, essentially maths. He had FM (a bit like OPs DC he took history, economics, maths and FM) and said he did not envy those attempting the course without FM. They had quite a lot of catch up. Other social sciences will be similar. You can certainly study them, but if you are a talented mathematician with an interest in research you are best off taking FM. Nowadays data dominates.
DD, who is a medic, is finding similar. You can certainly be a medic without FM. However she is now taking a research orientated intercalation, and even though maths A level was required and there was an additional summer school, it is still pretty stretching. Her friend who did take FM as a fourth, and is a genuinely talented mathematician, is doing even more maths in an intercalation which involves understanding infection and immunity rates. (Wrong year - it is completely Covid dominated.)
Other social sciences will be similar. You can certainly study them, but if you are a talented mathematician with an interest in research you are best off taking FM. Nowadays data dominates.
DS' was comfortable with economics and maths for A level, but more worried about history and FM. There is a step up at A level. In the event he found FM way more interesting than anything at GCSE, whilst he found history, and the requirement for argument and evidence, harder that he had expected. If you start with four, you have room to drop one.