Three major starpowers
In the case of the Milky Way, the term Three Major Starpowers refers to the Hemmoian Federation, the Gigerdi Alliance, and the Bacterian Empire. It was used extensively during the Peace Station Era and especially aboard the Station itself.
Although there were other major governments in galactic politics at the time, these particular three civilizations were considered the prime movers behind the Peace Station Project as they had also been the participants of the First Bacterian War.
The popularity of the term decreased after the Peace Station Project with the foundation of the Inter-Stellar Alliance, which technically united the former adversaries. Other events that contributed were important changes in the political unity of the starpowers, such as the Hemmoian Civil War, the foundation of the IPA and the DHR, and the reappearance of ancient civilizations like the Great Lightness, which dramatically changed galactic power relations.