Indigo 3 is the location of the Blue Desert in the audio drama The Skull of Sobek. The desert is perfectly symmetrical, but in the centre is the Sanctuary of Imperfect Symmetry, a religious retreat which exists to counterbalance the desert's perfection.
Inter Minor and Inter Major features in the Third Doctor serial Carnival of Monsters as the location of the Miniscope.
Iphitus Yet another planet mentioned in The Greatest Show in the Galaxy.