I used to be firmly in the "KH3 will be for Wii" camp, because of the Wii's higher install base, the Wii's cheaper dev costs, Disney's liking for Nintendo, and the ability to pair the Wiimote with a Keyblade, but now I'm not so sure.
Outside of Guitar Hero and the Lego titles, most games sell better on ps3 than Wii, let alone ps3 and 360 combined. With Japanese devs seemingly returning to the ps3 more and more these days (Alterier Rorona, Ar Tonelico III, 3D Dot Game Heroes, etc.), and Square's heavy investment in ps3/360 development already, I'm beginning to think mutliplat ps3/360 is the more likely option for the series.
A KH3 on Wii would sell millions, to be sure, but it would probably sell a decent bit better on the ps3/360. The question is whether the disparity in dev costs would make a ps3/360 version worth it.
But then, DQX went to Wii. So who knows.