Throughout the film, Kristi carries around a stuffed killer whale. This is actually a stuffed version of Willy from the Free Willy series which was not produced until 1993. It is apparent due to the three black spots on the underside of the stuffed whale's chin. Paranormal Activity 3 is set in 1988 which would make it impossible for this toy to have existed.
The Barbie doll that can be spotted on the lower part of the screen during the rotating camera sequences throughout the movie is an early 2000s model, making it impossible for it to have stood in a 1988 home as depicted in the movie.
The 1980s video equipment room has a road case for modern electronic equipment. One is a CDJ (Compact Disc Jockey) road case. Portable professional CDJ equipment did not exist until the mid-to-late 90s.
While the film takes place in 1988, the home video tapes are presented in wide screen and high-definition quality. However, this is a necessary and deliberate change on the part of the crew, and not an error. Producing a movie with "authentic" quality based on the equipment that existed back then would make a horrible quality for it to be seen in theaters and the movie would be unusable to most theaters.