Once you receive a message from Cartman, head back to the Coon Lair and sit across him to start the quest. Shortly after, you’ll be able to choose your third class. Like before, consider getting the class that will complement your current skill set and equipped artifacts. The new available classes are Assassin (Brawn, Spunk) and Gadgeteer (Brains).
Once you select your third class, your backstory will be modified again and will serve as a tutorial combat for your selected class. Just follow Cartman’s instructions to learn about your selected class’ abilities and the mission will be complete. Don’t forget to modify your equipped powers through the Powers app of your smartphone afterward.