Lya III was the third planet in the Lya system. Lya III was the location of a Federation starbase. Vice Admiral Haden was based at the starbase on Lya III. (TNG: "The Defector")
After completing its mission at Angosia III, Lya III was the destination of the USS Enterprise. (TNG: "The Hunted")
Lya III was located in the Alpha Quadrant. (Star Trek: Star Charts, page 47)