Bio & chem go well together, so probably do both. Bio has more content to remember but it’s easier to do, Chem has less but it’s harder.
As for FM - do it if you want to do a maths degree, but otherwise, I wouldn’t bother. For many other courses it forces you to continue 4 subjects to A2 instead of dropping one, as FM isn’t counted as 2 full subjects.
You need to be good at maths, but you need to enjoy it if you’re gonna do well at FM. It’s not enough to just be good. You’ve got to want to do it.