Hi,
If you dont want to track material B, no need to include the same in BOM. Create BOM for the material A with material C as the component and maintain the quantity accordingly. If you are using material B for in case of inhouse production, then maintain 2 BOM for the material A.
1. BOM for material A with B & C components (Multi level BOM with components for inhouse prod)
2. Alternate BOM for material A with C as component (Single lvel BOM with component C for external procurement)
If you are dealing with multiple subcontract vendors, you may check the thread: Re: Is this Subcontracting Scenario or some other process
Regards,
AKPT