I took advantage of this timeshare offer but stayed the full 4 nights. If you search FT you will find multiple threads on this offer, so your post may get merged into one of those by the mod.

I personally wouldn't take the chance by leaving a day early, but if you actually do the timeshare presentation before you leave and don't actually check-out early (i.e. just leave and let them book your card on file for charges) then you should be fine.

Btw, the timeshare presentation is optional, but is strongly recommended by the hotel staff. It's only an hour of time and there really isn't a strong sell. Well worth it for the points they offer.