The Triplets are another three ghosts of the mansion. One day, while playing Arctic Explorers and Dinosaurs, they noticed they were getting awfully tired, and fell asleep. Alas, the room they'd been playing in had a carbon monoxide leak, and when they woke up, they discovered that they somehow had died.
The Triplets then promptly decided to use their newfound undead status to go scare the cook and steal the fresh-baked brownies. As far as they're concerned, being ghosts only makes the adventures better.