Lists by TheFirstWave
Voice acting is included, but made-for-TV films, straight-to-video releases, and video games are not - only Hollywood theatrical releases. Actor does not have to appear in all films of a series, or even 3 films consecutively to be counted: As long as he or she has been in 3 or more films in 2 different series, it counts.
Actors are listed chronologically by when they joined this club - based on release year of their second threepeat, which is included in parens.
I'm not as knowledgeable about films from the 70s and earlier. Did I leave anyone off, or make any other mistakes? Please comment below if so. Thanks!