This help file is for the "simple" buy 2, get 1 free type discount.
If you need something more customised/complex - please look at a custom discount. Also adding a free item to cart automatically
To have a buy 2 get 1 free, it needs to have 3 items in the cart, as the customer will take 3 items out of the shop. So the condition is where 3 items are purchased.
However, if the free item is from a different category, that the conditional/reference items, then you can just specify 2. Because, buy 2 oranges get 1 apple free, then once you have 2 oranges, then apples become free. So min quantity of oranges is 2, target item is apples, and get 100% off 1 item of apples, per reference quantity.
If a more complex offer is needed as below, use the custom discounts area: