Sega updated their recent financial statement and removed the mention of Sonic Generations for 3DS.
http://www.nintendoworldreport.com/news/26364
Sonic Generations’ future on the 3DS is now in doubt after Sega Sammy issued a revised earnings report.
A 3DS version of Sonic Generations was listed in the original earnings report, but no longer appears in the updated document. Sega had also listed the game as heading to the PC, but that has also been erased.
Sega may be correcting a mistake by editing out the 3DS listing or is waiting to officially announce the game at a later date. NWR will continue to follow the story.
(http://i.imgur.com/hIQd1.jpg)