The three main elemental attacks in every Final Fantasy. For a lot of enemies it really doesn't matter which of the three you cast. Granted you get flare and whatnot further in... but when your black mage only has those three spells, which do you usually prefer if the enemy has no elemental weaknesses?
PS - Water doesn't count because it's usually a more powerful attack later on
PS - Water doesn't count because it's usually a more powerful attack later on