Hi teashop ,
What you're asking for will be possible with the upcoming Product Levels release. We're working on a "Produce items" functionality.
It will allow increasing the stock levels of product A by reducing the levels of its linked BOMs (raw materials B and C). Of course, activating this option will change the PL standard functionality. Selling products won't reduce their linked BOM stocks anymore with it active.
Best Regards,
José Andrés