Originally all three temples in the Cross Group looked much like the Temple of the Sun, the best preserved member of the Group.
"Chan-Bahlum pursued the triadic theme further in the design of the buildings themselves. In each temple, three doors pierce the front wall of an interior which is divided into an antechamber and three rear sanctums. In the central chamber of each temple his masons built the holy portals which opened into the Otherworld. These powerful foci of supernatural energy were set inside miniature houses--called by the Maya pib na or ""underground buildings"--built within the back chamber of each temple. While these little houses were only symbolically underground, they replicated in principle the real underground buildings of Palenque: the tombs of Pacal and other kings in pyramids which dotted the sacred landscape of the city."
Linda Schele and David Freidel, A Forest of Kings: The Untold Story of the Ancient Maya, p. 238-9