It's difficult to gauge fan hype/social media trending and advance a concrete idea of how a game or product will do based solely on that. Films, for example, are tracked prior to release using all sorts of metrics in order to get a sense of where the intended audience is leaning, and the tracking is (frequently) wrong, sometimes very much so. This happens in both directions: films overperform and underperform relative to expectations all the time.
I've learned to take all the anticipation surrounding this game with a nice spoonful of salt. Its audience is ultimately still a niche within a niche; a subsection of JRPG fans who actually want to have their gaming experience filtered through the lens of Disney tropes and archetypes (or, even nicher, people like myself who want to see them disrupted on some level). I think the general appeal of the Disney elements is also overestimated: my guess is the vast majority of adults who watch and enjoy animated Disney films/all Disney films do so precisely because they are easy popcorn entertainment that last only a couple of hours, and the idea of having to immerse oneself in the process of playing through those stories for what is comparatively an eternity therefore defeats the purpose. For people who already gamers and Disney fans, it's a no brainer, but again, that's a niche.
So the question is really, I think, will kids beg their parents to buy this game for them the way many of us begged ours to buy KH1 for us? That's hard to say. The "3" might warn a lot of younger players away, as well as the emphasis on how it represents the "epic conclusion" to a long-running saga. Or they might not care and pick it up for the recognizable and friendly Disney characters. I do think it's in a less optimal position than either KH2 or any of the portable titles in that it's much easier for someone to pick up a series on the second installment or a side story/prequel and assume they'll make sense of the content somehow: there's a sense that they're not too late to get on the ride or, if you will, bandwagon. Once words like "the end" start popping up, though, I think people are a lot more hesitant to, in the first place, get themselves in too deep with something that may not provide any meaningful returns for them and, also, potentially spoil the possibility of experiencing the story from the beginning (someday).
Through sheer marketing and exposure (and current gen accessibility), KH3 will obviously beat out the handheld installments, but I wouldn't be surprised to see it end up closer to KH2 than KH1's performance. KH2 had hype and marketing as well, and it didn't go all that far on it.