So I have been stalking my dates for a couple months now, and back in February had picked up two consecutive 1-night award nights for my upcoming stay in a month, was trying to get a third night after those nights. No luck up til now. I've been checking 1-night, 2-night, and 3-night combos. Well today, suddenly the full three-night combo I needed showed up on points! The other interesting thing is the two-night combo on the front end was also available, but not the two-night combo on the back end. Of course the point calendars all show these nights as unavailable. So I would keep trying 2 or 3 (or more) night combos if you are looking. Like I said, none of them had been available in the past months I've been looking, but suddenly a few combos were available today.

Available:
N1 + N2
N1 + N2 + N3

Not available:
N1 only
N2 only
N3 only
N2 + N3

Edit: Also, when I cancelled my original 2-night reservation, it DID go back into inventory on points.