The Hollywood reporter has just posted a picture of the third exclusive minifigure to be given out at the LEGO booth at SDCC 2014. The minifigure is Bard the Bowman from The Hobbit. The picture on the left is of a life sized model of Smaug the Dragon which will be on show at the event.
The other picture is a life sized model of Groot and Rocket from Guardians of the galaxy which will also be on display.
Brickset is also reporting how the exclusive minifigures will be given out - All the badges from attendees will be scanned in order to determine the winner in a lottery style giveaway.
I would imagine more on SDCC will be revealed every day leading up to the event.
All of the posts here are written by Ben