If you were really committed to watching the movies in order, you'd have to watch parts of each movie as there are flashbacks in most of them, but they work just watching them in the order where the majority of its action takes place. Although it takes place after Chapter 3, Insidious: The Last Key goes back further than any other movie in the series as it flashes back to Elise's childhood in 1953. Set nine years after the events of Insidious: Chapter 2, Insidious: The Red Door sees Dalton and Josh confront the demons that continue to haunt them from The Further.