Lorraine Warren, played by Vera Farmiga in the The Conjuring movies, died a natural death aged 92 on April 18 2019. She had been a head consultant on all of The Conjuring projects, and an avid follower of the series.
The working title of the movie was plainly The Conjuring 3. However, later it was changed to The Conjuring: The Devil Made Me Do It as a direct reference to the infamous 1981 Arne Johnson trial.
When Father Kastner (John Noble) says that he prefers to keep his satanic objects locked up in his basement "like keeping guns off the streets", Ed and Lorraine exchange a meaningful look. In The Conjuring (2013), Ed said the exact same thing to a reporter while giving a tour through his basement full of cursed artifacts.
This is the second movie about the Brookfield murder that happened in 1981. The first was The Demon Murder Case (1983).
James Wan remarked that with The Conjuring 3 they wanted to get out of the haunted house setup which was the subgenre of the previous two films. The Conjuring: The Devil Made Me Do It (2021) takes a major turn in this regard.