Hi Daar,
Create BOM for Material A with child as B and maintain in BOM under basic data -special procurement key as 50 - Phantom assembly .
Create BOM for B with child as C.
In the above case when BOM of A is exploded it will trigger requirement for C only as B is phantom assembly. Similary when BOM of B is exploded it will trigger requirement of C only.
Thanks & Regards,
Ramagiri