Most of the scenes in the trailers are not in the actual movie which angered many fans of the series. According to Inside Pulse, the production team behind this had enough leftover footage to assemble two completely different versions of the same movie; fifty-percent of the footage was what they used to create the TV spots and theatrical trailer.
(at around 5 mins) Towards the beginning of the film, Julie takes a picture of young Katie standing in the driveway shortly before a group picture is taken. The photo being taken appears in both Paranormal Activity (2007) and Paranormal Activity 2 (2010) as an indication that the demon has started haunting them.
(at around 1h 12 mins) The symbols that appear behind the pictures in Grandma Lois's house are simplified versions of the Pentagram of Solomon (inverted) and the Triangle of Solomon, symbols depicted in the "Lesser Key of Solomon" and used to summon and constrain demons in classical Goethic Theurgy.
(at around 4 mins) The scene with Kristi's loose tooth was real. "Little girl shows up on set and says that she's got a loose tooth," Ariel Schulman said. "We say, 'Film it, work it into the scene.' Can't fake that."
As with Paranormal Activity (2007) and Paranormal Activity 2 (2010), Paramount Studios once again utilized paranormal investigator/researcher, Christopher Chacon, who is recognized as one of the world's foremost authorities on paranormal phenomena, to internationally promote and publicize this third installment. Chacon also works in the entertainment industry as a writer, director and producer.
The camera seen on screen during most of the film is actually a tiny Hi-Def camera hidden inside the body of a VHS camcorder; for the first few weeks of shooting Smith did not have a viewfinder to see what his footage would look like. "So what they had to do was they built, or they put in a screen, a little monitor in the viewfinder that was transmitting the video that it was recording in the body of this old VHS tape into this monitor. So I was basically watching a little monitor in the viewfinder. And it was like, week six when they figured that out."
The film takes place in September 1988, with certain dates highlighted graphically. One highlighted date is 9/10/88, but then the events skip ahead past 9/11, a storytelling decision that was very important to the filmmakers. "Out of respect," Joost said. "Never gonna do that."
Very little set dressing was done on the house from Paranormal Activity 3: "We would walk into [some of the potential locations] and it would be like they were frozen in the 80s," said Peli. "We wouldn't even need to do too much to them. So, sometimes you get lucky. [...] We didn't want to throw the 80s thing in your face too much. We didn't want it to make it hugely like, 'Look, it's the 80s!"
To help celebrate the DVD/Blu-ray release of Paranormal Activity 3, Paramount held a contest in which two contest winners would be flown out to LA to attend a special viewing of the film at the house in which it was filmed. The winners were picked based on trivia knowledge of the franchise and, in this instance, were flown in from Michigan - treated to an private viewing of the film, dinner and a few special "guests" including Oren Peli.
The trivia items below may give away important plot points.
at around 1h 18 mins) In the static at the end of the movie, there is a frame that shows Dennis' and Julie's bed on fire. This is presumably the start of the house fire that is referred to in both Paranormal Activity (2007) and Paranormal Activity 2 (2010).
Though they spent the great majority of their time behind the scenes, Joost and Schulman actually make cameos in the film. Schulman was the ghost underneath the blanket, "Catlike reflexes," he cracked. "Very agile." Joost's cameo is similarly mysterious, though at least part of his body is visible - even if you can't tell.