If you want to offer a deal like "Book 3 Nights and Pay for 2 Nights Only" then here are the steps that you need to follow.
Create a new rate
If your rates are dynamic, not every night costs the same, then you can simply create a rate depended on your Base rate, and discount it by either 33% for 3=2 deal, 25% for 4=3 deal, 50% for 2=1, etc., by changing the Relative adjustment in the Rate settings. Please find more information below:
Restrict the rate
We would recommend you to create a new rate, restrict it for minimum and maximum 3/4/2 nights depending on the deal. You can find more information on how to restrict rates here: