"The Three" was a nickname given to a triumvirate of Praetors that secretly dominated the government of the Romulan Star Empire in the mid-to-late 23rd century.
History and specifics
Until the year 2276, the Three were Eveh tr'Anierh, Ahrm'n tr'Kiell and Urellh tr'Maehhlie. In the 2270s decade, the Three cemented their power base with antagonism against Havranssu and Ship-Clans descendants throughout the Empire. They initiated rsearch into the Sunseed technoloy to propogate ion storms and started research into enhancing Romulan mental powers at Levaeri V. After the taking of the USS Intrepid II to harvest Vulcan brain matter, the Federation responded by shutting down and stealing the research. After their excesses led to the outbreak of open war with both the Federation and Klingons, the Free Rihannsu movement moved against them and the Romulan Grand Fleet. With the installation of Empress of the Romulan Star Empire Ael t'Rllaillieu, and the restoration of Gurrhim tr'Siedhri to the Praetorate, the Three were captured and removed from power. (TOS - Rihannsu novel: The Empty Chair)