She serves Professor McGonagall, Professor Flitwick, Hagrid and Cornelius Fudge, and is invited to join them for a drink. She is unhappy that there are Dementors in Hogsmeade, as they have searched the pub twice and scared her customers away.
Rosmerta finds it difficult to believe that Sirius Black went over to the Dark side, as she knew Black when he was a boy. When Professor McGonagall asks her if she remembers Black's best friend, Madam Rosmerta remarks how you never saw one without the other, eventually revealing that Black's best friend was James Potter, Harry's father.
Madam Rosmerta isn't impressed that Professor Moody insists on drinking from his hipflask whilst he visits the Three Broomsticks, instead of buying a drink from the pub, and glares at Moody as she collects glasses from the tables around him and Hagrid.
Madam Rosmerta was placed under the Imperius Curse by Draco Malfoy. It was she who gave the cursed opal necklace to Katie Bell in the bathroom of the Three Broomsticks, and poisoned the bottle of mead before sending it to Professor Slughorn at Christmas, believing it was meant for Dumbledore. She and Draco communicated via enchanted coins.