<P> To keep the show's title plausible, the show's head writer, George Tibbles fashioned a three - part story arc in which an orphaned friend of youngest brother Richard (Chip, played by Stanley Livingston), Ernie Thompson (played by his real - life brother, Barry Livingston), awaits adoption when his current foster parents are transferred to the Orient . Steve offers to adopt Ernie, but faces antagonism from Uncle Charley, who finds Ernie a bit grating, and forecasts major headaches over both the boy and his dog . It also transpires that a law requires a woman to live in the home of an adoptive family . A likable female social worker supervises the case, and the Douglases speculate that Steve might marry the woman, to make the adoption possible, but they both agree this is not reason enough for them to be married . The family also does not need to hire a housekeeper, since Uncle Charley already has things running smoothly . The family soon appears before a judge who researches the law, and determines that its intent is to ensure a full - time caregiver is in the household . With Charley meeting that role, and having had a change of heart about Ernie, Charley assents to a legal fiction declaring him "housemother" to the Douglas family . </P> <P> While the three sons were always central to the storyline, several major changes took place by the late 1960s . In the spring of 1967, the ratings for the series began to sag and My Three Sons finished its seventh season in 31st place in the Nielsen ratings . It was decided that the 1967 - 1968 season would bring the program not only a new time slot, but also new storylines to spice up the ratings . In the fall of 1967, CBS moved My Three Sons to Saturday night at 8: 30 pm . In the season - premiere episode, "Moving Day", the Douglas family and Uncle Charley relocate from the fictional town of Bryant Park in the Midwest to Los Angeles . Robbie (Don Grady) marries his classmate / girlfriend, Katie Miller (Tina Cole). Tina Cole, in fact, had appeared in different roles on three previous episodes of My Three Sons: "House For Sale" from the fourth season (February 13, 1964), "The Coffee House Set" from the fifth season (November 19, 1964), and "Robbie and the Little Stranger" from the sixth season (February 17, 1966). At the end of the 1967 - 1968 season, the ratings had improved from the previous year with the series placing at 24th in the Nielsens . The following season, the newlyweds discover that Katie is pregnant, and she gives birth to triplets named Robert, Steven, and Charles . Although originally played by sets of uncredited twins, these babies were played uncredited by Guy, Gunnar, and Garth Swanson . The most familiar triplets in the show's last two seasons are played by Michael, Daniel, and Joseph Todd . </P> <P> The following year in the tenth season, 1969 - 1970, Steve remarries, taking widowed teacher Barbara Harper (Beverly Garland) as his wife; she brings with her a 5 - year - old daughter, Dorothy "Dodie" (Dawn Lyn), so Steven now had a stepdaughter whom he also subsequently adopts . (Dodie is wary of Steve at first, believing that he wants her to simply forget her late father, until he explains that he wants her to always remember and love him, but since he's no longer alive, Steve wants to raise her in his place, and hopes she'll come to love him also .) Also, the last year - and - a-half of the series feature fewer appearances of both Don Grady and Stanley Livingston . Grady's character was written out of the show at the end of the 11th season, which allowed for his wife Katie and their triplet sons to remain within the Douglas household the following season (as a structural engineer Robbie was working on a bridge construction in Peru). Chip and his teenaged wife Polly (Ronne Troup) (who eloped after Polly's disciplinarian father refused to sanction their marriage) move into their own apartment . </P> <P> At the end of the 1970 - 1971 season (the show's 11th year), My Three Sons was still garnering healthy ratings . By the spring of 1971, it had finished in 19th place . A 1971 television pilot with Don Grady and Tina Cole called Three of a Kind, then retitled Robbie--about Robbie, Katie, and the triplets moving to San Francisco--was filmed but not picked up as a series . The final episode of the 1970 - 1971 season, "After the Honeymoon", actually set up the premise for this pilot . The guest stars were Richard X . Slattery and Pat Carroll, who were featured as the landlords of the apartment block into which Robbie and Katie move . However, Don Grady had informed the producers of his intention to leave the series and pursue a new full - time career as a composer, which he ultimately did . </P>

Who does chip marry on my three sons