|Native Species:||"Old Ones" (extinct)|
This was one of the planets where all humanoid life went extinct; four others were Cheron, Kataan, Scalos, and Zetar. (TOS: "Wink of an Eye", "Let That Be Your Last Battlefield", "The Lights of Zetar"; TNG: "The Inner Light")
In or shortly before 2261, famed medical archaeologist Roger Korby led an expedition to this planet, consisting of at least himself and his assistant, Dr. Brown. Korby sent messages through 2261, and then ceased to communicate.
Korby revealed that the civilization of Exo III had fled the surface as the planet began to cool, eventually exchanging their open, free society for an increasingly mechanistic one. He also found the immense android Ruk, still tending the machines of the ancient civilization, a people Ruk called the "Old Ones". Even Ruk no longer remembered how long he had been doing this work.
Among the machines of the Old Ones, Korby discovered a still functional android duplicator. He planned to use this technology to slowly replace living people with androids. His idea of utopia held that androids could be programmed not to experience any of the negative feelings or express any of the negative behaviors of living beings. Korby claimed that such androids were superior to the living beings they replaced, but the observations of Kirk and Christine Chapel suggested that Korby was incorrect in this view.
Speaking to Ruk, Kirk learned what had happened so long ago. The Old Ones had built better and better androids, but became fearful of them and began to turn them off. Their survival threatened, these machines exceeded their programming and murdered their builders.
It was ultimately revealed that all of the survivors of the Korby expedition, including Korby himself, were androids. In various conflicts that developed between the androids, all were destroyed. (TOS: "What Are Little Girls Made Of?")
Presumably, Exo III was located in the Alpha Quadrant.
In the script of "What Are Little Girls Made Of?", the surface of Exo III as seen from orbit is initially described thus; "The coloration of the planet suggests extremely low temperatures on the surface. However, not totally snow-covered." The script details a closer view of the surface by calling it, "A semi-twilight landscape of barren wasteland buried in deep snow; jagged ice peaks loom in b.g. against a dark and sinister sky." 
The recreation of Quark's Bar which was located in the Star Trek: The Experience attraction in the Las Vegas Hilton Hotel served an alcoholic drink called a Warp Core Breach that was reportedly served with ice crystals from Exo III.