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 death of Crassus and the jealousy of Pompey against Caeasar eventually led to a civil war and Caesar's victory.
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. Conflicts began brewing from the start. While Antony was the most militaristic, Octavian was a better diplomat. Eventually, with the death of Antony and the luxurious imprisonment of Lepidus, Octavian was the sole remaining leader and went on to become the first Emperor of Rome, changing the period of Rome from the Roman Republic to the Roman Empire.