Alpha III was the third planet in the Alpha system. This planet was famous for being the site where the Statutes of Alpha III were written. It was home to the well-known Tribunal of Alpha III, who wrote important laws including the Statutes. (TOS: "Court Martial")
Alpha III was located in the Alpha Quadrant. This planet shared the Alpha system with Alpha V. The system's primary was a G-class star. This star had a magnitude of +3, which was 10 times brighter than Sol. Alpha III was a Federation member. (Star Trek: Star Charts, United Federation of Planets I & III)