2 years ago on Gem, we were also 3 adults in 1 cabin: 1 and 2 had the drink package, 3rd did not.
On the first days of the cruise, we tried to go according the rules, and the 3rd passenger would try to buy their own drinks with their own card (and their own money), but the bartenders would not accept her card, and would instead put her drinks on the cards that had the drink package.. π
I donΒ΄t think her own card was charged ever on the times we were ordering together. They would always just hand it back and use the cards that had the drink packaged.