Denius III was the third planet in its star system.

In 2365, the Federation conducted an archaeological dig on Denius III, headed by Dr. Ramsey. His team discovered an artifact of Iconian manufacture, and Ramsey allowed Captain Varley of the starship USS Yamato to carry the artifact from the dig. A study of the device on aboard the Yamato revealed a starfield, which allowed Varley to locate the planet Iconia. (TNG: "Contagion")

This planet was only mentioned in dialogue.