As of my knowledge cutoff in September 2021, there are only two "Max Crumbly" books: "Max Crumbly: Middle School Mayhem!" (2020) and "Max Crumbly: The Legend of the Lost Legend" (2021), both written by Rachel Renée Russell. There is currently no information about a third book in the Max Crumbly series.
Rachel Renée Russell, the author of the Dork Diaries series, has not announced a release date for a third Max Crumbly book. Please check the official website or social media pages of the author or publisher for the most up-to-date information about upcoming releases.