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.