Costing Sheet / Cost Group Question

Hi:

If I have an item called ItemA as well as two component items called ItemB & ItemC that make ItemA. I also setup a Materials Cost Group called MATL.

In assigning my inventory items to the cost group, do I assign ItemA, ItemB & ItemC to the MATL Cost group or do I assign only ItemA to the MATL Cost group when I am going to calculate the costing sheet?

Steve