No, this is NOT Kingdom Hearts 3. But it leads into it. The next game after this will be KH3, and it will also spell the end for Xehanort.
Also, Neku's only in this cuz he was designed by Nomura, so it makes sense. I doubt they're gonna start adding in whatever they can. Plus, this takes place in the realm of sleep, so I doubt Neku will be a new recurring character.
More than likely, he remains within Traverse Town for the entirety of the game.
DusK: The game is 358/2 Days, and they're developing this to be on par with Birth by Sleep, and be the final transitional title between 2 and 3.