OP - I should point out that not all exam boards offer AFM. I can tell you're not talking about OCR, because they only seem to have 17 modules (C1-4, FP1-3, S1-4, M1-4, D1-2) but the only boards that actually offer AFM are Edexcel and OCR MEI.
I'm assuming that you'd be applying for maths with 3 maths A-levels - that would probably be the only subject that would accept just those 3, but they might not take kindly to it. Although it's good to be focused, they'd want to see a bit of variety. And what happens if you get bored of doing maths? You wouldn't have an alternative to fall back on.