Judith Light starred on the soap “One Life to Live” for several years, then “Who’s the Boss?” and one other sitcom the name of which I can’t recall, but I think it ran at least two years…
Vivian Vance, redux: kawliga is correct and Vance will be removed from the list.
Michael J. Fox: Does not qualify. IMDb’s listing is incorrect, he only guest starred on Trapper John, M.D., in an episode called “Brain Child.” Leo & Me, Palmerstown, U.S.A. made it anywhere near 80 episodes.
Ken Berry: Does not qualify. Just looking at IMDb you would think he does. Both The Ann Sothern Show (92 eps) and Dr. Kildare have enough episodes. Mayberry, RFD only had 78 episodes, but reaches qualification if you include his character’s episode on The Andy Griffith Show. However, according to Ken Berry’s official web site he was only on Ann Sothern for one season. It doesn’t make specific mention of Dr. Kildare but lists that time as one of “semi-regular” appearances, so I can’t think that his role was that significant, or that he did three seasons of work.
Judith Light, redux: Her third series was Phenom which does not qualify.
Don Adams: Questionable. Could not find reliable information on duration of Tennessee Tuxedo. It was on the air for almost exactly three years. This makes it about even odds as to whether it hit 80 episodes (assuming “standard” season). Check it Out could be his third season, joining two Get Smart series and Inspector Gadget; unfortunately it was a Canadian series and is disqualified.
Ja’Net DuBois: Does not qualify. Her stints on Love of Life and The Wayans Bros. were only two years each.
Sherman Hemsley: Does not qualify. Amen did much better than I remembered, and of course The Jeffersons counts, but Dinosaurs only had 65 episodes.
Sally Struthers: Does not qualify. All in the Family/Gloria are good enough. As is The Pebbles and Bamm-Bamm Show. As mentioned, though, Dinosaurs only had 65 episodes. Tailspin reached the same mark.
Scooby-Doo - “Norville ‘Shaggy’ Rogers”. Superfriends - “Robin” and others. Josie and the Pussycats - “Alexander Cabot III”. The Adventures of Batman - “Robin” and others. Transformers - “Cliffjumper” and several others. Battle of the Planets - “Mark”
Scooby-Doo, Superfriends and Transformers all had at least three seasons.
Casey Casem: Qualifies. But I ain’t too comfortable with it. I’ll let the following explain, but note that Transformers is the only individual series to reach 80 episodes.
Shaggy
[li]Scooby Doo, Where Are You? (41 episodes)[/li][li]The New Scooby Doo Movies (22 episodes)[/li][li]The Scooby Doo/Dynomutt Hour (24 episodes)[/li][li]Scooby’s All Star Laff-A-Lympics (16 episodes)[/li][li]Scooby’s All Stars (16 episodes)[/li][li]Scooby and Scrappy Doo (16 episodes)[/li][li]The Richie Rich/Scooby Doo Hour (21 episodes)[/li][li]The Scooby and Scrappy Doo Puppy Hour (13 episodes)[/li][li]The New Scooby and Scrappy Doo Show (23 episodes)[/li][li]The New Scooby Doo Mysteries (13 episodes)[/li][li]Scooby Doo Mystery Funhouse (13 episodes)[/li][li]The 13 Ghosts of Scooby Doo (13 episodes)[/li][li]A Pub Named Scooby Doo (25 episodes)[/li][li]Dynomutt, Dog Wonder (appeared in 3 episodes)[/li]Total: 232 episodes
Robin
[li]The Batman/Superman Hour (17 episodes)[/li][li]The Adventures of Batman (same series as above)[/li][li]Super Friends (16 episodes)[/li][li]The All New Super Friends Hour (16 episodes)[/li][li]The World’s Greatest Super Friends (59 episodes)[/li][li]Super Powers Team: Galactic Guardians (10 episodes)[/li]Total: 118 episodes
Robert Picardo: Does not qualify. Only 64 episodes of China Beach.
Richard Dawson: Does not qualify. Being a regular on a game show has not counted so far. Hosting the show may be different, but Dawson was a panelist for The Match Game.
I think you need to give a pass to actors who did have three full seasons, even if they fall just under 80 episodes. Star Trek is a good example-79 episodes was 3 full seasons, and the OP did admit that the number 80 was somewhat arbitrary. Even if you’re gonna really nitpick Shatner for 79 ST eps, Nimoy would make it because he was in the original pilot-“The Cage”- ST’s 80th episode.