Teased last week, it was part of a viral marketing promotion that would unlock once the official Facebook page reached 1,776,000 posts, tweets and shares.
Assassin's Creed 3 will take place over a 30-year period in colonial America beginning in 1777.
New abilities are expected to feature in the game, such as climbing trees and hunting wildlife.
Ubisoft president Yves Guillemot has called Assassin's Creed 3 the "biggest in the series to date".
The game is also on course to become Ubisoft's most pre-ordered game of all time.
Assassin's Creed 3 will be released for Xbox 360, PC, PlayStation 3 and Wii U on October 31 worldwide.
> 'Assassin's Creed 3' first-look preview
Photo gallery: Assassin's Creed 3 screenshots: