The third part was the lunar module, which was actually supposed to land on the moon. And once the work on moon was over, it was supposed to get attached to the CSM module once again. The fourth part, the Launch Escape System, was not so important for the mission. Its only task was to protect the astronauts from an accident during the launch.
It works like that, If there’s an accident during the rocket launch, and the rocket catches fire, it safely ejects the astronauts and takes them away from the accident. But if everything goes well in the launch, then the LES is not needed and it is ejected and thrown away.
On 11th April 1970, since Apollo 13 launched safely and successfully the LES was not needed. 3 astronauts were on the spacecraft. Commander Jim Lovell, the pilot of the Lunar Module, Fred Haise, and the third, pilot of the Command Module, Jack Swigert.
Coincidentally, according to the original plan, none of these three were chosen for this mission. Originally, three different astronauts were chosen for this mission. But they all had different problems because of which they were not able to participate in this mission.
