You can add a coupon with type Buy X Get The Cheapest Free to handle this.
Buy X Get The Cheapest Free type requires setting a min qty, such as setting to 2 will mean BOGOF (buy one get one free), or set to 2 for Buy 2 get the 3rd free, and so on. In each case the free item is always the lowest cost item and is added as a dynamic lump sum discount to the order.
This also works with the in cart deals system so once one or more of the item is in cart, it'll advertise that you can get this deal until it applies.
You may also be able to use Quantity Break Pricing on your product to achieve Buy One Get One Free, 3-for-2 and other promotions involving the purchase of more than one quantity of a product to get a discount.
Quantity Breaks are set in Product Manager > select the product > Prices & Delivery tab
BOGOF's: Enter a quantity of 2 with a price that is half the normal price
e.g. Normal price £10 per product, on line 2 enter a quantity of 2 and a price of £5. Then when two products are purchased the total price is 2 x £5 = £10
3-for-2: Enter a quantity of 3 with a price that is 2/3rds of the normal price.
e.g. Normal price £9 per product, on line 2 enter a quantity of 3 and a price of £6. Then when three products are purchased the total price is 3 x £6 = £18
Add other multiples using the above examples to guide you.
Display A Banner Showing The Offer
To display a banner on the product image, on the product record > Main Details tab, scroll down and enter your banner text and banner colour code. This will display as a stripe across the top right hand corner by default. Tip: Keep the text length short as there is limited room available.
Testing! Always test your promotions to make sure they're working as you expect.