I'm not trying to bash you as I see you have a lot of correct information. However, at this current time you do have to look at it like a business. For one, Square Enix is doing downright horribly in finances, so badly that at this rate they will go bankrupt within a couple years according to an article posted on Kotaku.com today actually. You are absolutely 100% correct when you say the 360 does horribly in Japan, but Kingdom Hearts is not a Japan-only game. In the US, the 360 easily outsells the PS3 and with this recent PSN outage, the 360 is profitting even more (the Wii 2 we can't assume until a pricetag is set and more info is released). The 360 and the PS3 have roughly the same controller in terms of of layout with the only difference being one joystick and the D-pad switched on the controller. For this reason, porting KH3 to the 360 would be extremely easy as the controller can stay the same (the Wii 2 controller may have the same format as well but we don't know once again) and if they are doing that bad financially, why not port it? Not everyone in the US and Europe has a PS3 and 360, so for those who only have 360, they can get the game as well which increases Square Enix's profits for the game. Releasing the game on all consoles even if it has not been on it previously can basically only help Square Enix and at this point they need it. Yes, when they released Final Fantasy XIII on the 360 some fans boycotted it, however, overall Final Fantasy XIII sold well. The only difference between FFXIII and KH3 is that many people are demanding KH3 at this point in time, so for a fan to boycott the game purely because it is on the 360 is just stupid on their part. I have to admit, originally I was one of those people would have gotten very mad at Square Enix had they made KH3 for the 360, however seeing their trouble at the moment and because I want KH3 to be a big success so the series will continue, I do hope the release it for multiplatforms, all of them if they can.
Like I said I completely understand it as a business (coincidentally I work in marketing so I totally understand it's a business) and they might/might not (No one's 100% sure right now) release it on the 360. I just can't see it that's all just a personal opinion.
Also that is just current sales, not overall sold units in the US/NA with the PS3/Xbox 360 so we honestly don't know what Square-Enix decides to do till they do it.