Garrus and Liara are definitely returning with others depending on whether they survived. Cerberus is royally ticked off and after Shephard and the Normandy.
The third game follows on from the events in the Arrival DLC for Mass Effect 2 which will have Commander Shephard facing trial, but he's soon excused as the Reapers show up knocking down the door and invading Earth. Cerberus wants Shephard dead.
It's possible this is due to certain choices made by the player though, so Cerberus might not be hunting every player. Garrus and Liara return to the squad with others also showing up depending whether they survived the original or sequel game.
Multiple endings make a return and are decided by your actions and squad choices. BioWare promise there's a lot more RPG in Mass Effect 3, and greater freedom in shaping the player character with larger skill trees and more.
Multiplayer is not to be included with the third game, but could in a later instalment. Mass Effect 3 releases on Xbox 360, PS3 and PC later this year.