Threepeat

Three-peat is a term used primarily in American sports to refer to winning three consecutive championships. The term, a portmanteau of the words three and repeat, originated with the Los Angeles Lakers of the National Basketball Association, during their unsuccessful campaign for a third consecutive championship during the 1988–89 season, having won the previous 2 NBA Finals in 1987 and 1988 against the Boston Celtics and Detroit Pistons, but were swept by the Pistons in the 1989 NBA Finals. The term is a registered trademark owned by Pat Riley, the Lakers' head coach from 1981–1990, although it was coined by L.A. player Byron Scott immediately after their victorious championship defense against the Detroit Pistons in the 1988 NBA Finals.

In a comedic context, the same play on words, additionally incorporating the name "Pete", is known to have been used as early as 1930 on the radio program "Empire Builders!The episode of that program broadcast on December 29, 1930, featured a trio of singers dubbed "The Three Visiting Firemen: Pete, Re-Pete, and Three-Pete".

The Oxford English Dictionary credits an Illinois high school senior, Sharif Ford, with the earliest published use of the word in the March 8, 1989 edition of the St. Louis Post-Dispatch. Ford's quote uses the term in a sporting context and serves to provide a clear etymology as well:

The Lincoln High Tigers say they want to "three-peat". "You know, kind of like repeat, except doing it for the third time", senior Sharif Ford said.

However, Riles & Co., the corporate entity of National Basketball Association (NBA) coach Pat Riley, submitted in November 1988 a trademark application for the use of three-peat on shirts, jackets and hats. At the time, the phrase was being used by members and fans of the Los Angeles Lakers basketball team, of whom Riley was the head coach, regarding the Lakers' quest that season to obtain what would have been a third successive NBA championship. According to Riley, it was Laker player Byron Scott who coined the term in reference to the team's goal for that season.

...
Wikipedia