The premiere of Star Wars Rebels season 3 was promised at Star Wars Celebration Europe 2016, and indeed the first two episodes were shown to the fans attending. For Star Wars fans not so lucky to be in attendance, Lucasfilm did release a trailer for season 3 which provides a taste of the many things to look forward to this Fall. Ezra's haircut and lightsaber are not the only thing different in season 3, take a look:
Yes, we also get to see Kanan's new look post-season 2, the introduction of original Star Wars trilogy mainstay Wedge Antilles, more Maul, and lots of drama and action, but… what fans will most notice is the new threat brought on to menace our heroes: Grand Admiral Thrawn.
Before Star Wars: The Force Awakens hit the big screen, the people at Lucasfilm redefined the Star Wars universe's canon, chucking out many of the old stories and characters and re-labeling "Legends". Much to many fans' dismay this included author Timothy Zahn's popular Thrawn Trilogy of books which introduced the Grand Admiral to the post-Return of the Jedi era. Thrawn will be voiced by Lars Mikkelsen (Rogue One: A Star Wars Story's Mads Mikkelsen's brother).
It looks like they might be going all in on the character as well, since on the publication side, they are also publishing a book titled Star Wars Thrawn by Timothy Zahn himself. This book is expected out in April 2017.
Now if they could just give us more information on Ahsoka Tano's fate… (showrunner Dave Filoni only cryptically said "it's likely you haven't seen the last of her" when asked about her at the Star Wars Rebels panel, curse you Filoni!!!)
Update: There's also a poster for season 3!