The shorter the length of the waitlist the more likely that it will hit. I would be shocked if a three week waitlist came through - that most likely would mean that three separate people decided to cancel three separate weeks, there were no other waitlist requests, and no one simply booked one or more of the nights that came available after the cancellation but before all three weeks were available.
I've never even tried to waitlist a week. I have been able to put together a week a night or two at a time over the course of two or three months.
All that said, if you really, really want to go to a specific resort and have the points to spare, book every night in the same villa type that is currently available and put in waitlist requests for the missing nights.