This is possible to do, and there are a couple of options available. Whichever option you chose, we are happy to advise and assist you.
Set up a discounted rate
The channel manager would need a rate that is 33% off your nightly rate, so the total price for 3 nights is the same as a 2-night stay, or thereabouts.
For example, you are selling Room Only at 100.00 per night we would need a rate that is 33% off this price, so we are sending 67.00 as the "3 for 2" daily rate. With this calculation, the total amount would be 201.00 - as close to 200.00 we can get.
The reason why we need to have a nightly price is that we are unable to communicate "free" nights to the booking sites.
You would also need to ensure that you set a minimum and maximum length of stay restriction of 3 nights on the rate. This will stop anyone booking for 2 or 4 nights.
Set up a promotion online
The better option is to set up a promotion online. Most major booking sites offer "free night" promotions based on your mapped rates. Reservations will then deliver onto the mapped rate with the discount already applied.
They will also advertise it as an offer, driving more business. For more information on this, please contact your OTA's directly