Most all the programs I've seen define a stay as consecutive nights regardless the number of check-ins/outs. So breaking up a paid stay with a reward stay should not matter. Of course some properties may mess this up and you'll get what you want, but don't count on it.

Now if you're feeling generous, book the second night in your SO's name and let them get credit for one of the three nights. At least then someone will get credit for the room, and maybe someday you can book a room under their account.