L-374 III was the uninhabited third planet in the L-374 system. This system was near and in the same sector as the L-370 system, and was near the Rigel system.

This was one of two planets destroyed in the planet killer attack of 2267; the other was L-374 IV. The other two planets in the system, L-374 I and L-374 II, were spared. In 2267, the entire crew of the Federation starship USS Constellation, consisting of 400 men and women, was beamed down to this planet for safety against the planet killer which was attacking the Constellation with her Captain, Commodore Matt Decker still aboard the starship trying to outrun the machine. When the Constellation was disabled, the Doomsday machine turned onto L-374 III. Severely damaged and with transporters offline, there was nothing Decker could do to save his helpless crew. As a result, the doomsday machine destroyed the third planet and blasted it to rubble and debris, and Decker's entire crew were killed. (TOS: "The Doomsday Machine")

This planet was only mentioned in dialogue.
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