The Pure Ones, also referred to as the Three Brothers, were the Garou tribes of the Americas (called the Pure Lands): the Uktena, Croatans, and Wendigo, or the Elder Brother, Middle Brother, and Younger Brother, respectively.
The term was first used after the first European Garou arrived with the first white settlers, to differentiate between the native Garou and the strangers. Because of the utterly destructive effect of the European colonial effort on American tribal nations, many Garou of the three American tribes called those Garou who came with the settlers "Wyrmcomers".
The "Pure Ones" called themselves pure because they maintained the purity of their lands after they crossed the Bering Strait along with their Kinfolk millennia ago. They drove off Wyrm beasts, sealed Banes, and taught their Kin the ways to live in harmony with the Grandmother - all of these things that the European tribes apparently neglected.
In the modern nights, only the Wendigo insist on the name, as a means of separating themselves from the rest of the Garou Nation who stole their lands. The Croatans sacrificed themselves to banish the Eater-of-Souls, and the Uktena have bred with other indigenous lines to expand their horizon - and consequently, some of the more aggressive Wendigo maintain that the Uktena are no longer Pure.