Okay, I had time on my hands, and now I trained these three characters to level 90, and 91. Theoretically you can already go to Kefka's Tower when you have these three characters (due to Celes' travel, and the new airship). Of course that's just theoretically speaking, 'cause you're most likely to weak to go into there right away.
But, after doing all the sidequests in the World of Ruin, and training the three first characters you get in this world, can it be done practically, besides theoretically: beating all monsters and bosses encountered in this final dungeon with only three characters. :O
I shall try it out, and wish me good luck, 'cause somehow I got a great feeling about this.
Comments, anyone? ^^