Complex sentence: In the third film, Stan, the father of the family, is the secondary antagonist, and is shown to be a neighbor of the Heffley's and has a childhood rivalry with Frank, that they appeared to have gotten over as adults.
Split into simpler sentences: In the third film, Stan, the father of the family, is the secondary antagonist, and is shown to be a neighbor of the Heffley's. He had a childhood rivalry with Frank, that they appear to have gotten over as adults.