Stay three nights and pay for two, at least that’s what Starwood says when you book by February 14 using your MasterCard. The actual dates of travel need to be by April 3. You must use the code M3C for rates that include room only or M3B for rates including breakfast.
This is only for participating hotels in Asia and Australia, although the list appears to be quite comprehensive.
The Points Guy suggests that you might be able to combine this discount with the fourth free night you get when booking with the Citi Prestige credit card, which is also a MasterCard. If true, that would mean four nights for the price of two since each free night is credited separately.