After the Trans Lunar Injection burn the Command Module disconnected from the Stage 3 unit, rotated 180 degrees and docked to the Lunar Lander Module which was stored in the compartment shown above that
was between the 3rd & 4th stages.

The compartment split apart into 4 leaves before the Lunar Module was extracted.

The Lunar Module was stored in a compartment between Stage 3 and the Command Module

Saturn V Lunar  Module (LeM) Storage