On the same wikipedia article (taken with a grain of salt) it says:
The aft storage, compartment, behind the crew couches. This housed the 70 mm camera equipment, the astronaut’s garments, tool sets, storage bags, a fire extinguisher, CO2 absorbers, sleep restraint ropes, spacesuit maintenance kits, the 16mm camera equipment, and the contingency lunar sample container.
That seems to me like there could very well be enough room for a third if some of the equipment was left out. If they had a secret space program, maybe said program already had a foothold on the moon? And if they had a base on the moon, all of the equipment would not be necessary for the journey there, thus allowing Astoronaut #4 to squeeze in the backseat.




