In your example a cart wide 33% discount would be applied to each item. So the CD and both songs would get 33% off.
There is not a way to setup a discount that applies to a group of items, so you would not be able to setup a discount that will only work with CDs. You could make product specific discounts, but the limit is then for that specific product, so for the discount to work, the buyer would need to have 3 of the same CD in their cart.