Haven’t thought of any of my own yet, so I will comment on the contributions of others:
Lee Majors: In addition to the three series mentioned by bibliophage, Majors had a fourth series make it three seasons, Owen Marshall: Counselor at Law, there were only 70 episodes, however.
Bill Cosby: I Spy barely qualifies Cosby because it only had 82 episodes. The first Bill Cosby Show (1969) likely does not meet the 80 episode requirement, though it lasted around three seasons.
Kate Jackson: In addition to bibliophage’s listing she starred in Dark Shadows, qualifying show; however, she was not on it for three seasons.
William Shatner: If you include the the animated series, William Shatner played Captain Kirk in 101 episodes of something. You do have to include Rescue 911 to get three series as Barbary Coast was his next most successfull series with just 14 episodes.
Harry Morgan: Pete and Gladys only had 70 episodes so it does not qualify on its own, Morgan’s character, however, was a spin-off of December Bride which had 146 episodes.
James Garner: Does not qualify. Maverick had 124 episodes (though Garner was only in 55), and Garner then revived the character in Bret Maverick in 1981 (another 18 episodes). The Rockford Files had 123 episodes (and several TV movies). Unfortunately, none of his other series (Nichols, Man of the People, God, the Devil and Bob) lasted even one season.
Carroll O’Connor: Does not qualify. All in the Family (210 eps.), Archie Bunker’s Place (97 eps.), and In the Heat of the Night are his only successful TV series. As mentioned the first two are the same character. However, with over 450 TV episodes, he must be one of the most successfull in that count.
Betty White: Uncertain. Her first show, Life with Elizabeth, lasted '53-'55 so it may have 80 episodes. None of the THREE The Betty White Shows lasted more than a year. Mama’s Family had 130 episodes, but White was only on for the first two seasons. Her clear qualifications are The Mary Tyler Moore Show (168 eps.) and The Golden Girls (180 eps. with another 24 eps from Golden Palace).
Bob Newhart: Does not qualify. The second The Bob Newhart Show and Newhart both qualify, but none of Newhart’s other shows do.
Bill Bixby: Uncertain. The Courtship of Eddie’s Father only had 75 episodes. My Favorite Martian had 107 episodes and The Incredible Hulk had 85, that give him two. Trying to find a third we have a show he hosted called Once Upon a Classic; I don’t know how many episodes there were, but it ran from 1976-1979 so it may have hit 80. In addition to those named, he starred in another qualifying show (though for only one season), The Joey Bishop Show.
Ted Knight: Does not qualify. Two qualifiers are The Mary Tyler Moore Show and Too Close for Combfort. The closest to a third was voice work on New Adventures of Superman, in which he was the voice of Perry White. Only 70 episodes, though.
Markie Post: Does not qualify. Heart’s Afire had only 54 episodes. The Fall Guy and Night Court do qualify, though.
Howie Mandel: Just barely qualifies as Bobby’s World had exactly 80 episodes. The Howie Mandel Show (talk show) may have made it to 100 episodes considering the higher number of episodes for talk shows.
Don Knotts: Qualifies with not three, but four series. Search for Tomorrow was a soap opera on-air from '51-'86; Knotts was on from '53-'55. Five seasons on both The Andy Griffith Show and Three’s Company are obvious, but Knotts was also the voice of Vice Principal Bone on Doug
Dick Van Dyke: Qualifies with The Dick Van Dyke Show, The New Dick Van Dyke Show (different character), and Diagnosis Murder.
Richard Boone: Does not qualify. Neither The Richard Boone Show or Hec Ramsey reached 80 episodes.
Shelley Fabares: Qualifies with four, Annie Oakley, The Donna Reed Show, One Day at a Time,and Coach.
Elinor Donahue: Father Knows Best and The Odd Couple are the only two shows that qualify.
John Astin: Does not qualify. Gomez Addams is a character with more than 80 episodes, but only if you include the animated series as well.
Yeardley Smith: Does not qualify, but barely. Brothers and The Simpsons are clear winners, but Herman’s Head only had 72 episodes.
Ted McGinley: Does not qualify. Only appeared in two seasons of The Love Boat. Happy Days and Married…with Children qualify though.
Mary Tyler Moore: Does not qualify. Only The Dick Van Dyke Show and The Mary Tyler Moore Show qualify.
Tony Danza: Does not qualify. Does not have a third show to join Taxi and Who’s the Boss?. Family Law may change that some day, however.
Robert Urich: Does not qualify. Not one of his shows has made it to even 80 episodes (Spencer: For Hire only had 66).
Mike Farrell: Does not yet qualify.
Pernell Roberts: Does not qualify. FBI: The Untold Stories was only on for two seasons. It is unlikely it reached 80 episodes.
Charles Siebert: Does not qualify. He has only on Search for Tomorrow for two years.
Howard Hesseman: Does not qualify. He was only on One Day at a Time for two seasons.
Ok, to sum up we have:
Gavin McCloud
Michael Landon
Julie Kavner
Harry Morgan
Shelley Fabares
David Hasselhoff
Lucille Ball
Dick Van Dyke
William Daniels
Don Knotts
Howie Mandel
Lee Majors
Heather Locklear
Bill Cosby
Betty White and Bill Bixby are uncertain.