Let’s observe the steps of the action configuration.
Please open Admin Panel > Promotions > Shopping Cart Price Rules (click ‘Add New Rule’ button) > Actions.
Step 1. Step 1. Choose ‘Percent Discount: each 1st, 3rd, 5th with 15% 0ff after 5 items added to the cart’ field.
Step 2. Use ‘Each Product (step)’ field to indicate a discount step. If you indicate ‘2’ like below, it means that each second item, that meets the rule conditions, will go with 20% discount.
Step 3. Use ‘After N’ setting to indicate the number of ‘N’ items. If you specify 3 like here, it means that a customer will have to buy 3 ‘N’ items to get a 20% discount for the each second item, in this case for the 5th, 7th, 9th item and so on.
And here's the rule which will give each second item after purchase of 3 for free
Here's the diagram for this rule
This is how the result will look like in the cart