Returning items with discounts that have multiple items as criteria is always hard. This is so tied to how the underlying ERP, OMS and receipt system all work. Your ERP might be better off with not having a product being 0 cost, but what would be your approach if you buy these 3 products and then return 1 of the items? Does the customer get to keep the NOK 30,- reduction for each item, or do you need to adjust the price and maybe charge the customer more at the return? Does the receipt management support changing of prices after a receipt has been generated or do you need to create a completely new receipt? Etc...

My point is this tricky and too hard for a framework to make a decision that will affect all of their commerce customers. And making this change is too small anyway to help answer all above question. So long story short, no, it's not possible to set up this behavior out of the box. But epi is very extensible, so you always have the possibility of implementing it yourself :)