triumvirate

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Government by triumvirs.
• noun The office or term of a triumvir.
• noun A body or group of triumvirs.
• noun An association or a group of three.

from The Century Dictionary.

• noun The office or magistracy of a triumvir, specifically of one of the ancient Roman groups of triumviri.
• noun Government by three men in coalition.
• noun A group of three men in office or authority; specifically, in Roman history, either the coalition (First Triumvirate) between Pompey, Julius Cæsar, and Crassus, 60 b. c., which controlled the Roman world for several years, or that (Second Triumvirate) between Mark Antony, Octavian (Augustus), and Lepidus, 43 b. c., which overthrew the republican party and ordered the second proscription.
• noun A party of three men; three men or three personifications in company or forming one group; also, a trio or triad of any kind.

from the GNU version of the Collaborative International Dictionary of English.

• noun Government by three in coalition or association; the term of such a government.
• noun A coalition or association of three in office or authority; especially, the union of three men who obtained the government of the Roman empire.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A group or association of three, especially three statesmen.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a group of three men responsible for public administration or civil authority