In Assassin's Creed III, the opening historical sequence begins in 1754.
The game initially places players in the role of Haytham Kenway, whose story begins in London before moving to the American colonies. This opening introduces the Templar perspective and establishes events that lead into the later colonial storyline.
Connor, whose Mohawk name is Ratonhnhaké:ton, becomes the central protagonist later in the game. His main story unfolds during the American Revolution, but that is not the year in which the historical sequence begins.
The dates 1775 and 1783 are associated with major stages of the Revolutionary War and its aftermath. They are therefore plausible alternatives, but they do not mark the opening year of Assassin's Creed III's historical narrative.