Hello,
Create BOM for A material with components as B and C as per the quantity required. Defined Material A as phantom assembly so demand will be directly pass on to components B and C.
Create purchase order for material A do good receipt you will have the stock for A.
Then create disassmbly purchase order through CO07 here use A component with positive quantity and B and C with negative quantity. After confirmation of order you will have stock for B and C and A material stock will get consume.
Best Regards,
R.Brahmankar