@ChannelLightVessel, assuming your DD is good at Maths and has the grades to be accepted to do 4 A levels including FM, that is quite common - there is more content but with 3 A levels that really do link closely together, they reinforce.
Doing Maths in year 12 and Further Maths in year 13 is not so common, but my son's sixth form does it that way and it means that he already has an A* in hand, and he's applying to study Maths at university, and I think that's quite a big positive for him, and it means they're only studying for 3 exams at a time, but have extra teaching time for the Maths and Further Maths (other subjects, Physics and Philosophy, are over the normal 2 year course).
I would assume students who pass Maths but are advised or they decide that they don't want to carry on to FM are still doing 2 other A levels. And students who carry on to FM but drop a different subject can still come out with 3 A levels.
Also, DS1 has 3 university offers based on 3 grades including his Mat.s. So if he needs to prioritise he can concentrate on FM and Physics.
Your DD and her sixth form can and probably will assess and decide whether she will go on with 4 A levels at the end of year 12, before university applications in year 13. I think going from 4 to 3 at that point puts students in a better position than going from 3 to 2.
Most 6th forms do require higher GCSE grades for entry to study 4 A levels and/or to take Further Maths