n. The involvement, support, or cooperation of three parties, especially three political parties.
2001
Tom Daschle, who will become Senate majority Leader once Mr Bush's tax bill has passed, was triumphant. 'This will be America's first 50-49-1 Senate. What does not change with this new balance of power is the need for principled compromise. This is still one of the most closely divided Senates in all of our history. We still face the same challenges. Bipartisanship — or I guess I should now say tripartisanship — is still a requirement.'
1992 (earliest)
You cannot help but favorably compare, at this time, our state government to the gridlock in Washington. I beg our Senate, House and governor to continue this tripartisanship.
This word has been popping up all over the place for the past few days in the wake of U.S. Senator James Jefford's decision to leave the Republican Party and sit as an independent. Strangely, although the adjective tripartisan dates all the way back to 1959, the noun tripartisanship seems to date only to 1992.