Free upgrades are extremely rare on points reservations, and generally only happen if there's a maintenance issue with the room that's been pre-assigned for you and there's nothing else open for them to move you to. With timeshares, you get what you book.
Adding to that, grand villas are limited in number and extremely popular with members, so there's often a waitlist for them. It's unlikely that at a 3-bedroom villa would be available on your check-in day, even if you were willing to pay the difference.