A triumvirate is a governing body comprised of three members. The most famous instances of this occurred in the Roman Republic.
The First Triumvirate
Formed in 60 BC, it was comprised of Julius Caesar, Pompey, and Marcus Licinius Crassus and took place 'under the table'.
The Second Triumvirate
Formed in 43 BC, this was the first legally formed triumvirate in Rome. It was comprised of Octavian, Marc Antony, and Marcus Aemilius Lepidus.