The reason behind Happy Death Day 3 delays has been clarified by director Christopher Landon. Happy Death Day was released in 2017 to positive reviews and box office success. The film follows Tree, a college student caught in a time loop that forces her to continuously relive the day of her murder until she finds her killer. A sequel, Happy Death Day 2U, was released in 2019, garnering similar reviews while not performing as well as the first film at the box office. Still, director Christopher Landon has plans for another sequel in the franchise.
https://screenrant.com/happy-death-day-3-development-delay-christopher-landon/