I am gold and registered for the Fall megabonus

Earn: 5,000 Marriott Rewards bonus points for every third Visa-paid stay you make – up to a total of 25,000 points

What is a "stay"? Do they count the entire resevation as one stay or individual night as one stay? I stayed there last week for 3 nights but only credited one stay under "promotion progress". Can I work around this if I pay each night using different Visa card?

Thanks.