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