So apparently (no offical confirmation) a Sonic Mega Collection 3 is in the works according to Sega of Japan's offical Sonic Channel on Youtube (It's at the bottom of the channel descrption on the left): http://www.youtube.com/user/sonicschannel
It's TBR 2010 for the PS2, Wii, PS3, and Xbox 360. I serached the web for a game list, but came up empty and will update the thread once one becomes available. Maybe it's Mega Collection+ and Gems Collection combined? Speculate!