The way my TA explained it to me, in order to get 3 free rooms you would actually need to have 18 rooms booked. You actually have to have a 6th room booked to apply the free night too, even though technically the credit could be applied to your master room. Additionally, we were told that all of the numbers will be figured out on the day that all payments are due, which should be spelled out in your contract. I am getting married there in May and my final bill is due April 9th, including all of my guests rooms. On that day we will know how many rooms are booked and how many credits we will get, of course, after that point last minute bookings will count as well. Are you using a travel agent?