Which version of Actinic do you have? I don't think this facility was available in the non business version (or the developer edition)
Look under View | Discount and Surcharges Settings.
You will need to assign the products on offer to a unique group (they can only appear in the one group) ... you can set the reward type .. for 3 for 2 offers set ...
Based on = quantity
Trigger = 3
Reward type = % off cheapest
Product group = select the one you created above
Reward amount = 100 (this is 100% ie all the cost)
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