In order for KH3 to outsell FFXIII, FFXIII would have to be an outright failure salewise unless you lot are dreaming that KH3 will produce 6mil+ sales despite the fact that none of the past games achieved this.
KH2 was released in 2006 wasn't it? Massive userbase... but the game sold a good 2mil less than KH1 because we all woke up and realized that big userbase does not = big sales.