A triumvirate is a group of three people. A duumvirate is a group of two people.
The word is often used to mention a historical event or in talks of government. The word troika is Russian for triumvirate, and the term was used in the Soviet Union after V.I. Lenin died, and Joseph Stalin, Lev Kamenev, and Grigory Zinoviev teamed up to gain power of the government. in the Soviet Union, troikas were usually considered to be three main people in the government: Party leader, head of state, and head of government.
In the Roman Empire in 43 B.C.E., after Julius Caesar died, a triumvirate made up of Marc Antony, Marcus Lepidus, and Octavian formed to rule the Empire. This was known as the "Second Triumvirate"; the first was Caesar, Crassius, and Pompey.