"One" is a song written by Harry Nilsson and made famous by Three Dog Night whose recording reached number five on the U.S. Billboard Hot 100 in 1969 and number four in Canada. The song is known for its opening line "One is the loneliest number that you'll ever do". Nilsson wrote the song after calling someone and getting a busy signal. He stayed on the line listening to the "beep, beep, beep, beep ..." tone, writing the song. The busy signal became the opening notes of the song.

Who wrote the song one by three dog night? OK. Let's think. My stream of consciousness:
To answer the question, consider the following: "One" is a song written by Harry Nilsson and made famous by Three Dog Night whose recording reached number five on the U.S. Billboard Hot 100 in 1969 and number four in Canada. The answer is Harry Nilsson.