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This topic might have a better name.

Even if in most of the serial it is referred as the generic "third planet", in the last episode it is called "Planet of Evil", that sounds more specific. "Third planet" should stay as a redirect.

Talk about it here.

You may be looking for the TV story.
The Planet of Evil, also called third planet and the Valley of the Shadow, was an artificial planet used as a base by the Shadow in attempt to obtain the sixth segment of the Key to Time.

The Shadow positioned the third planet between Zeos and Atrios so neither could be seen from the other. It was destroyed by Romana I and the Fourth Doctor when the missiles fired by the Marshal of Atrios's Command Capsule were rerouted into it. (TV: The Armageddon Factor)