Tutorial for configuring BuyX GetY

I am looking for a tutorial or assistance on how to setup this. 
I have really tried on my own but all just ends in a mess. Nothing becomes right in the checkout.

I want simply this:

In one selected product category - Customer selects three products in this category and when in the checkout, after applying a coupon code, the cheapest of the three products will be discounted with 100 %.

So simply, select 3 products and pay for 2.
(Shipping should not be altered.)

Joomla 3, Viruemart 3.

Kind Regards
/Dobromir

Many thanks for the rapid response. 
It looks like it works. :-)