I don't believe so - it looked to me like the rates quoted were for these specific listed packages. If you go to the reservation request page here it doesn't specify a 3 day minimum, and I am sure if you emailed them they would quote for a shorter time (except perhaps around Christmas and New Year).

That said, if your aim is to visit it for health benefits, I would suggest that you try to go for more than 3 days - it takes a while to start to destress. I prefer to go for 7 nights, and if I can't make the time, then certainly for 5. If you just want to visit a nice resort, then of course even a couple of nights is OK. Let me know if you have other questions.