Following the death of Heather O'Rourke in February of 1988 after she finished her work on the film (April-June 1987), it was the decision of director Gary Sherman to temporarily shelve the project during its post-production phase. However, due to the amount of money that had already been spent, MGM insisted that the film be finished and released as scheduled for June of 1988 or they would find someone else to do it. Apparently, after the film was given a PG rating by the MPAA in November 1987, the studio had already decided to have Sherman re-shoot the ending with more graphic scenes, in order to "up" the rating to PG-13. Planning for this re-shoot began in December 1987 and continued into January 1988, but was temporarily put on hold when O'Rourke died Feb. 1. The re-shoot (which used a stand-in for Heather) eventually took place in March, and the film was then "re-edited" and given a PG-13 by the MPAA in April 1988. Director Sherman would later claim that no such "re-shoot" took place, instead insisting that Heather died before they could film the "original ending" and that the current ending using the body double was what they hastily threw together when forced to "finish" the film by MGM. However, he is contradicted by at least six other people who also worked on the film who confirmed that the original ending was in fact filmed before Heather died and that the re-shoot of the ending took place after her passing. These people include producer Barry Bernardi, actor Kipley Wentz, assistant editor Jeanne Bonansinga, composer Joe Renzetti, special effects makeup artist Doug Drexler and the man who provided the voice for the Rev. Kane, Corey Burton.
At the beginning of the film, the characters mistakenly believe that the weather outside is cold. When they descend from the upper floors to the ground level however, they find that it is in fact quite warm. This phenomenon of weather varying from the upper to lower floors actually does occur at the Hancock Center due to the building's height. Residents often call the lobby doormen before leaving their apartments to find out what conditions are like at ground level.
After filming of the scene where the cars chase Patricia and Bruce, the car's explosion set the entire set on fire, almost taking a crew member and a few cameras he was rescuing. When Heather O'Rourke showed up for filming the next day and heard about the incident from director Gary Sherman, she was relieved that no one was hurt. She then asked Sherman, "Did you get the shot?"
Although much of the film is set in Chicago's John Hancock Center, the shopping area and parking garage as seen in the film do not exist in the Hancock Center. The shopping area (especially the escalators seen immediately before the art gallery sequence) is across the street, in the Water Tower Place shopping mall. The parking garage is definitely not the Hancock Center's, it was filmed in a high rise dual tower complex called "Oakbrook Terrace" in a suburb west of Chicago.
Although there was an internet rumor that Jerry Goldsmith was originally contracted to score this film but quit due to budget cuts (and then supposedly used his "unused" P3 score later in The Haunting (1999)), this rumor is untrue. Goldsmith was unhappy with the results of Poltergeist II: The Other Side (1986) and did not have an interest in doing the third film. Also, it's clear that MGM did not want to spend the extra money they knew it would cost to hire Goldsmith, considering that "Poltergeist III" was being made on a lower budget than the last film. Ultimately, "Poltergeist III" was scored by Joe Renzetti, who director Gary Sherman recommended, having worked with Renzetti previously on his other low-budget movies.
Craig T. Nelson, who played Steve Freeling in the Poltergeist movies, went on to play the title role in the TV series Coach (1989), which began preproduction in 1988 when this movie was being released, and ran for an impressive 8 seasons, from 1989 to 1997. It's what he's most known for, along with the Poltergeist movies. They are working on a Coach reboot with him reprising his role.
The Light that Dr Lesh talked about in Poltergeist (1982) that appeared at the climax of Poltergeist II: The Other Side (1986) and Tangina mentions at the end of this film may derive from war stories that pilots claim to have seen in combat.
There were originally plans for Craig T. Nelson to return for a Poltergeist 4, but the death of Heather O'Rourke, the ensuing media scandal about the Poltergeist curse that have overshadowed pretty much everything else about the movie, and the disappointing box office returns from Poltergeist III (1988) dissuaded the producers from continuing.
The producers were granted permission to use the John Hancock Tower in Chicago for shooting providing that none of the building's residents would be disturbed. The sixty person crew took four weeks just to figure out the logistics, and ultimately the tenants never even noticed that the film was shooting there.
The trivia item below may give away important plot points.
In the original ending that was scrapped after Heather O'Rourke's tragic death, when Patricia jumps through the glass pane into the apartment, she finds Carol Anne, Donna, Scott, Bruce and Tangina frozen and dying. She then also becomes imprisoned in ice and gets attacked by Kane and her evil mirror reflection who want the necklace. Patricia tries to repel them and declares unconditional love for her family, but trips over frozen Tangina and falls to the floor. Suddenly, Tangina frees her arm from the ice and grabs the necklace. She convinces Kane that she is the one who can take him to the other side, not Carol Anne. Kane puts his hand on the necklace, but instead of ascending, his face cracks and he explodes. The blast frees everyone, but annihilates Tangina and causes a violent thunderstorm. Patricia, Carol Anne, Donna, Scott and Bruce finally leave the mirror dimension. Carol Anne sees a reflection of smiling Tangina in the mirror who waves at them and sheds a tear. With the shot of a rising morning sun, the movie ends.