The third member of the Aztec Triple Alliance was Tlacopan. The alliance joined Tlacopan with Mexico-Tenochtitlan and Texcoco in 1428, creating the political foundation of what historians commonly call the Aztec Empire.
Tlacopan was a Tepanec city-state west of Lake Texcoco. It had originally been subordinate to Azcapotzalco, but after the alliance defeated Azcapotzalco, Tlacopan became one of the three victorious powers. Tenochtitlan and Texcoco were the stronger partners, while Tlacopan remained the alliance’s junior member.
The alliance was not a centralized nation-state in the modern sense. Its members coordinated warfare and shared tribute from conquered territories, but each city retained its own ruler and political identity. Historical accounts often describe the tribute division as favoring Tenochtitlan and Texcoco, with Tlacopan receiving the smallest share.
Azcapotzalco is the most tempting wrong answer because it was the defeated Tepanec power whose fall helped create the alliance. Tlatelolco, meanwhile, was a neighboring Mexica city closely linked to Tenochtitlan, but it was not one of the three founding members.