Assassin's Creed 3's multiplayer mode is being developed by Ubisoft Annecy, located in France. The news comes from the Twitter profile of an Ubisoft Annecy employee who lists his description as “Lead Game Designer on Assassin’s Creed 3 Multiplayer @Ubisoft - Annecy”.
Ubisoft Annecy developed the multiplayer for both Assassin's Creed: Brotherhood and Assassin's Creed: Revelations. Aside from AC games, the studio also worked on Splinter Cell: Chaos Theory's multiplayer.
Assassin's Creed 3's single player is being developed by Ubisoft Montreal.