Quick answer: The player who draws a 3 drinks.
What Does 3 Mean in King's Cup?
Three means “me.” The drawer takes the agreed action, then play moves clockwise. This rule has no lasting effect and does not create a special role.
Common 3 House Rule
Use one point, one sip of any beverage, or a quick movement such as a clap.
House rules differ by group. Confirm the meaning before the first card is drawn, allow anyone to choose a non-alcoholic alternative, and never pressure a player to participate.