The planet's atmosphere was within safety limits by Federation standards. Its gravity was 1.1 of Earth's, to which it also used to bear a remarkable resemblance before its primary began to dim 500,000 years ago. (TOS: "What Are Little Girls Made Of?")
As the star of the Exo system began to gradually dim, the average surface temperatures on Exo III eventually fell down to one hundred degrees below zero over the course of 500,000 years. The Old Ones, a humanoid species, fled their surface dwellings to the dark underground caverns to escape the inhospitable surface conditions.
The species exchanged their open and free society for an increasingly mechanistic one. The Old Ones built over time more and more advanced android servants. They gave the androids a sense of pride in their abilities and a desire to survive. The Old Ones however became at some point fearful of their creations and began to turn them off. As their survival was threatened, the androids bacame able to exceed their programming. They murdered their creators in self-defense, causing the extinction of the Old Ones.
In 2261, famed Federation medical archaeologist Roger Korby was leading an expedition on this planet with his assistant Dr. Brown. Korby sent messages back to the Federation through 2261. Around that time, the expedition encountered the android Ruk, still tending the machines of the ancient civilization centuries after the extinction of the Old Ones. Among the machines, Korby discovered a still functional android duplicator. Korby had at the time lost the ability to use his legs and was freezing to death. He created an android duplicate of himself before his body died.
When communications with Korby were lost, two further expeditions were sent to Exo III in an effort to locate the Korby expedition, but both failed to find them.
In 2266, the USS Enterprise, under the command of Captain James T. Kirk, traveled to Exo III in a third attempt to learn the fate of the Korby expedition. Android duplicates of Korby and Brown contacted the Enterprise as it arrived on orbit. The crew later on discovered that both Korby and Brown had died on the planet five years ago along with the rest of the expedition.
The android duplicate of Korby attempted to convince Kirk that the technology of the Old Ones should be used to secretly replace living people in the Federation with android duplicates. Since the duplicator could imprint the memories and the consciousness of the people being duplicated to the androids, the Korby-android was convinced that people would in time embrace the chance to live forever in android bodies. Kirk and Christine Chapel were not convinced, as the android duplicates had no sense of remorse or ability to experience genuine emotions. All the androids on the planet were destroyed in various conflicts that developed with them during the mission. (TOS: "What Are Little Girls Made Of?")
In 2268, the planetary profile chart of Exo III was displayed in one of the science labs aboard the USS Defiant before the ship was pulled into the mirror universe. (TOS: "The Tholian Web"; ENT: "In a Mirror, Darkly")
This planet was originally to have been named "Exo IV". Kellam de Forest found fault with this choice of name. He made a suggestion, included among a series of research notes (dated 9 May 1966), that the planet be renamed "Gamma Exo IV". However, the name remained "Exo IV". In another series of notes (dated 11 May 1966), de Forest recommended, "The first number could be dropped."
In the revised final draft script of "What Are Little Girls Made Of?", the surface of Exo III as seen from orbit was initially described thus; "The coloration of the planet suggests extremely low temperatures on the surface. However, not totally snow-covered." The script detailed 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 caves of Exo III were represented with a set built on Desilu Stage 10. (To Boldly Go: Rare Photos from the TOS Soundstage - Season One, p. 81)
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.
According to the novella "The Worst of Both Worlds", the mirror universe James T. Kirk of the ISS Enterprise destroyed the last surviving member of the android society (presumably Ruk's mirror counterpart) on Exo III, who had likewise turned on their masters and killed them.