It would appear that the Sonic game with double the Sonic is, in fact, coming to Nintendo 3DS, as already semi-announced by, well, Sega itself. But Shinobi? Well, that's good news too. Thanks, Nintendo Power magazine!
Sonic Generations for 3DS is the cover story for (what is allegedly) the next issue of Nintendo Power. That game has only officially been announced for the PlayStation 3 and Xbox 360, but 3DS and PC versions were hinted at by a recent Sega financial report.
Joining Sonic Generations (and Mega Man Legends 3) is another "classic revival", Shinobi. Exactly what that entails - is it a side-scroller? Is it a 3D action game a la Shinobi for the PlayStation 2? - I don't yet know, but seemingly soon we all will. (It's worth noting that resume scavenger "Superannuation" recently spotted a mention of Shinobi 3DS in development by Griptonite Games.)
Who's getting really excited for E3?!
Nintendo Power Confirms Sonic Generations For Nintendo 3DS [Sonic Stadium]