Base on the example buy two get one free... I found that if i add more than three or more item the value is equal to zero,

FIRST SET

Item1 = 15
Item2 = 15
Item3 = 0

the 4th item/5th item should have a price and for the 6th item should be discounted,

SECOND SET

Item4 = 15
Item5 = 15
Item6 = 0

repeatedly and so on....

The logic is number of item split by number of given quantity.

The promo must have an option like limit to (1,2,3...) set only or unlimited set.

Hoping that you've got my idea.