I don't know the 3 day cost (although I am curious for our Sept Cruise).
But just so you know...They will keep your pass in a file at the front desk of the spa. Filed under your last name or room number (I forgot which) then whenever you want just go check in - they will give you a towel and robe - you leave your key with them and are set.
When you are done you stop back by the desk and they return you keys.
They have tons of towels both in the Rainforest room and in the locker rooms.
I didn't know this and was a bit confused at first when they wanted the pass (which I was carrying with me) and my key. And was really confused when they didn't give the pass back to me once I was done. Once the process was explained I was good. So, I hope this helps.