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 StarTrek.com, if the planet was not part of Archer's Degra ruse, Maarek III was a M-class planet possibly located in the Delphic Expanse.[1](X)