3 for 2 offer

Im trying to get a 3 for 2 offer working on certain products in a group. If a customer orders any 3 products from a group of 6 then they should get the cheapest one free. Im not sure if this module will do this?

I can get things working to a degree when setting the conditions group values to different values but it adds the discount with only one or two products in the cart, not just 3. Setting the conditions group to the same value means the discount does not kick in at all.

Any help would be appreciated.

drupal 5, ubercart 1.x, uc_discounts 5.1x