It is surprising what a little number 3 next to the title will do. And not just in Kingdom Hearts. Portal 3, Half Life 3, (tried to think of a non Valve one, but it is 4 AM and screw you brain). Video games just have a love/hate relationship with the number 3.
Assassin's Creed 3, Super Mario Bros 3, DBZ BT 3 Sequels are easy to think of but not all of them have 3 in the title.