Purchasing combined items from a supplier

Supplier is delivering several item numbers, but some item numbers must be combined when purchased.For example item A must always be combined with items B and C.

When a buyer is entering a purchase order (either item A, B or C) there must be a signal that combined items must be purchased as well, preferable with the correct quantity relations.

Try to look at using Free Goods, but that does not work fine in combination with the pricing. Any one an idea?