Battle of Z
At some point after the events of the defeat of Majin Buu, Frieza, Cell, and Kid Buu team up together and manage to infiltrate the Hyperbolic Time Chamber and utilize it as their base. They also manage to gain a version of Vegeta (who still acts and looks the way Vegeta did during the Vegeta Saga) as part of their team. The Z Fighters arrive at the Hyperbolic Time Chamber and enter, confronting The Bosses; with Kid Buu and Vegeta using their base forms, and Frieza and Cell using their Final and Perfect Forms respectively. The Z Fighters successfully defeat their foes and send them back to Hell.
While they are not referred to as "The Bosses", in the first mission of the Saga of the Demon God Demigra, Villainous Mode Frieza, Cell, and Kid Buu are revived and appear as a team, working to defeat Future Warrior and Goku.