Holly definitely planned it to be a triology because of all how certain things were mentioned in the first book that came to play in the third one. The duct tape killer was mentioned in the first book by pip’s friend during camping scene. Andie’s school journal photo from the first book mentioned metting HH at 6pm or something, i.e Harriet H from the third book. Jason bell leaving the party for an hour or so because of the alarm that went off in his warehouse or something. I think there were more but I can’t remember them right now.