You have to make a combined warehouse for B and C. Just one warehouse with 2 locations.

It is important that you also create extra stock operations:

   *You must have 2 receptions (one for B and one for C)

    *Also 2 return operations (one for B and one for C)

    *2 POS operations are needed too (one for B and one for C)

The reception operations are important because if you only have the combined one, then when you purchase products and receive them, then you will have to make an internal transfer to put them in the right location. By having both reception operations you can set the location at the purchase order.

The separate return operations and the separate POS operations are important because even though you need the POS sale to discount the product from any of the locations; when a customer returns a product it must go to the right location without having to do an internal transfer. 

You have to configure the return operations inside the POS operations and also the source location (it must be the combined warehouse/stock) for both POS's

Once you have all this on the inventory app, you can go ahead and configure the POS operations for each POS (B and C)