How do you model a promotion which is triggered on a sum of 3 articles with hybris 6.*?

Dear friends,

My promotion reads:

For a sum threshold of € X, made on 3 promotional articles, receive a free gift article.

Here is an example: Qualified products are A, B and C. When added to the cND when their total is € X, then receive a gift. Note that X is the sum of the qualified articles and not the total sum of the cart.

Any ideas how to do that with 6.*?

Kind regards, Atanas