Free good determination

Hello gurus,

I'm customizing free goods, and I found a question:

I want a free good 3x2, buy 3 and pay 2, depending the hierarchy(ok, easy to customize), but for all the materials of the sales order, it means if you have 3 materials (diferent mat number) with the same hierarchy, one it's FREE.

¿it's possible customize this behavior?¿via user exit?¿program?

it's mandatory the generation of the free item category.

Because the standar SAP free good work per item cat no per sales order.

Thanks in advance.

Javier