theyre the 3 characters that are required for you to have to complete the journey. that is,you start out with celes, edgar helps you fix figaro castle, and setzer helps you find the airship falcon. after you get falcon, you can land on top of kefka's tower...you dont have to get anything else(characters, espers, etc.) in kefka's tower, you have to split up into 3 teams, but, if you dont do any other side quests after getting falcon, youll have just one person per team! (very difficult
) i mean, i suppose if they were at level 99....they might stand a chance...maybe...so anyway, i hope that explained why...sorry for the long post