Promotions - 3 for 2 - on whole category

The suggested requirement is that if the customer buys three items from one category they get the cheapest of the three items for free!!!

This is no simple 3 for 2 on one single product. And it is no fixed price off offer… Therefore i cannot see how it is possible. Also the fact that this is on a whole category means i would have to setup inter relational rules across multiple products and this would get very very confusing!!!

Any ideas on how i could do this or at least something similar?

I have tried setting up:

when order exceeds: 2 items…

and selecting all the products in one category and setting quantity to 0 on all of those.

But it requires that all items are purchased even if quantity is set to zero…

Do i have to add the products as individual conditions using the or statement inside a single box?

HELP!!! Thanks, N