In some disappointing news, Sega of America has stated to Game Informer, that they have "decided to not attend E3 with [their] own booth this year".
Sega's decision stems from needing to focus on its big headquarters move to southern California. With many of their major AAA releases coming later in 2015 or in 2016, the company plans to focus on those major announcements after their move.
This is certainly not the best news for fans of the company's properties but Sega did add that they "will be collaborating with [their] various business partners for this year’s E3 show."
So we'll likely see some Sega games at other companies' booths, just no dedicated Sega booth.
It should be noted though that recently acquired Sega subsidiary, Atlus, will be at E3 with their own booth.