Who was the adopted son on the U.S. sitcom "My Three Sons"?

The adopted son on the 1960 - 1975 U.S. sitcom "My Three Sons" was Ernie Douglas (Barry Livingston). Ernie as the Douglas' adopted son joined the household in 1965. Prior to being adopted, Ernie Thompson was a young friend of the Douglas family who is eventually adopted by the Douglas clan.

When this show first aired, widower Steven Douglas (Fred MacMurray) learns that 'boys will be boys'. He finds that he will at times struggle to raise his sons: Mike, Robbie, and Chip (Tim Considine, Don Grady, and Stanley Livingston respectively). Without the help of their mother, Steven has to be a clever, compassionate, solicitous and a loving father. Assisted first by the boys' maternal grandfather, Bub (William Frawley), and later by his brother, Uncle Charley (William Demarest), Steven Douglas does what he can to guide all the boys as they grow and mature. He and the family face a major change when Ernie Thompson becomes Ernie Thompson Douglas.