Three-peat is a portmanteau of the words three and repeat.

It is used either as a verb or noun used in American sports to refer to winning a third championship in a row.

The OED 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; the quote serves to provide a clear etymology for the term 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.

But Snopes.com (http://www.snopes2.com/legal/3peat.htm) reports that Byron Scott, then a basketball player with the Los Angeles Lakers, coined the term in 1988 as the Lakers started a season that might have ended in a third straight NBA title. Shortly thereafter, Lakers head coach Pat Riley trademarked the term for use on shirts, hats, and other apparel.