Nomura didn't say that KH3 was a prequel, he said that KH3 would be in another time, so it could also be in the future, where a lot of people have grown stronger of heart and therefore have keyblades. BUT Nomura also said that these are just thoughts, and could never happen, just like KH3 is still not confirmed and is a thought. but if the possible KH3 is out, and it is in another time, that means they would have to make an secret ending or make an KH4, to tell us what is in the letter Sora received