Maarek III was a planet mentioned by Jonathan Archer in 2153 in a deceitful ploy.

He told Degra that a fellow inmate in a Xindi-Insectoid prison had used it as a staging area while smuggling dilithium. It was described as "beautiful, with beaches that [went] on for thousands of kilometers." (ENT: "Stratagem")

This planet was only mentioned in dialogue.
Given the nature of the mention, it is unclear if there really is a Maarek III. The Star Trek Encyclopedia (4th ed., vol. 2, p. 7) described Maarek III as an "imaginary planet".
According to, if the planet was not part of Archer's Degra ruse, Maarek III was a M-class planet possibly located in the Delphic Expanse.