Create a BOM for finished product of usage 5, add materials B and C as components. Go to material master of finished product (Material A), note down the Item Category Group from Sales org 2 Tab. Now go to tcode: VOV4, for the sales order type for that item category group check out if TAQ is in the manual entries allowed column, if it's not present add it to one of the manual column.
Now whenever you create sales order if the Material A to be sold as it is then use the item category group TAN ( or whatever you've been using)
Whenever you want to package Material B and Material C then use the item category TAQ in the sales order line item (enter it manually while entering Material A and quantity in the sales line item, else default Item Category will be copied). By doing this way the BOM will get exploded in the sales order itself and you can sell Material A with components as B and C.