- You may be looking for the unrelated Delta IV.
Delta III was the third moon of Delta Magna. (PROSE: Doctor Who and the Power of Kroll) It was a refuge for the Swampies, natives of Delta Magna forced to relocate there by the humans who had colonised their original homeworld.
This moon was small. It had weak gravity, at an escape velocity of 1.5 miles per second, and a thin atmosphere. Humans had released giant squid on the moon in an attempt to recreate the Swampies' original environment because their god was a squid. The moon was covered in swamps and bogs and was rich in methane. When humans discovered this second feature, they set up refineries and attempted to exterminate the Swampies, a move opposed by humanitarian organisations such as the Sons of Earth. The Swampies were determined to protect what they had left because they knew they had nowhere else to go if more refineries were set up and they knew they'd be forced out.
In the 51st century, the Fourth Doctor and Romana I found the fifth segment of the Key to Time there, in the form of the Swampies' god Kroll. K9 Mark II remained in the TARDIS, as the moon's marshy terrain would have made it impossible for him to move about there. (TV: The Power of Kroll)