[EDIT/UPDATE - never mind, I see now this is in the ' Commercial Help Wanted ', and I saw the other post]
This is only now available with 3.50, as per release notes: "Discounts. Allow a store owner to specify maximum product quantity which could be discounted. For example, you can have two products (the same) in the cart but only one of them will be discounted. It can be used for scenarios like "buy 2 get 1 free".
There is a new field "Maximum discounted quantity:" when discount type is "assigned to products"