Joanie, Richie and Chuck
Chuck was played by two different actors:
Gavan O'Herlihy .... Chuck Cunningham (1974)
Randolph Roberts .... Chuck Cunningham (1974-75)
http://imdb.com/title/tt0070992/
The first two seasons of the series also featured Chuck, the Cunninghams' eldest child and Richie's older brother. The character was little more than a superfluous one, as he was usually used in scenes where he was "on his way to basketball practice." In fact, Chuck was originally a student at the University of Wisconsin-Milwaukee on a basketball scholarship.
The character of Chuck Cunningham was written out during the series' second season, and no explanation was ever given. Chuck was never referred to; scripts from later seasons suggested the Cunninghams had two, not three children. However, in the sixth-season Christmas episode, "Christmas Time," Fonzie – recalling his first Christmas with the Cunninghams – tells Chachi that "Chuck was away at college." In a Happy Days reunion show from 2005, the cast mentioned that Chuck had won a scholarship to the "University of Outer Mongolia" to play basketball, as a sort of an inside joke to the cast and writers. Such a situation of a character disappearing with little or no explanation, and then subsequent scripts continuing as if the character had never existed has become known as the "Chuck Cunningham syndrome."
https://en.wikipedia.org/wiki/Happy_Days
[Edited to add info & references - McG]
