The Three Galaxies are an affiliation of advanced spacefaring civilizations which coexist peacefully. They're not a single government as much as a political alliance (perhaps analogous to the European Union on 21st century Earth) whose goals include ensuring the protection of their civilization by patrolling the universe and getting rid of young species which are deemed too dangerous and with potential to become a threat to them in the future.
The Three Galaxies' oldest members are the humanoid inhabitants of the planet Lanador, in the Small Magellanic Cloud. From there the alliance has spread over to the Large Magellanic Cloud and parts of the Milky Way Galaxy, including the Vega System, as the Vegans are one of their members. They currently encompass hundreds of species; from beautiful humanoids to tentacled walrus-like creatures.
- Have Spacesuit – Will Travel, by Robert A. Heinlein (1958)