I believe I read somewhere that the Washington dlc will come in 3 pieces. Ill look around for that info again to confirm.
and here we go this is from the official site.
We are pleased to officially announce the Assassin’s Creed III Season Pass!
The Season Pass will give you access to all five upcoming downloadable content packs which are slated for release within six months after launch. Assassin’s Creed III downloadable content will feature an all-new single-player campaign told through three episodic content packs in which players will experience an alternate history of the American Revolution dubbed “The Tyranny of King Washington”. As the revolution comes to a close, a new and most unexpected enemy emerges. Driven by the desire to secure the fate of the colonies, the Revolution’s greatest hero, George Washington succumbs to the temptation of infinite power a new King is born.
In addition to this all-new single player experience, the Assassin’s Creed III downloadable content will feature new maps and characters for an even more expansive multiplayer experience as well as other single-player missions
So looks like 3 out of the 5 is the Washington dlc I guess and the other 2 I would assume is multiplayer. Just my guess though from what ive read.