You will get all three games plus some add-ons such as online pass (see below). These games are excellent and one of the best story driven shooters made.
If you do not have the games already I suggest you get them. More than 75 hours of entertainment plus online play for Mass Effect 3.
Here are the key features (copied from the Gamersgate website):
KEY FEATURES
Includes Mass Effect 1, 2, and 3. Over 75 hours of story content, plus Mass Effect 3’s critically acclaimed multiplayer mode.
Create and customize your own character with his or her appearance and skills, then play your Commander Shepard through all three Mass Effect games.
Experience an amazingly rich and detailed universe where your decisions have profound consequences on the action and the outcome.
Build a personalized arsenal with powerful weapons complete with their own upgrades and modifications
Unleash devastating abilities and skills as you command and train an elite squad.
Take the war online with class-based, co-operative multiplayer in Mass Effect 3.
On PC, Mass Effect will include Bring Down the Sky and Pinnacle Station on disk. For Mass Effect 2, Cerberus Network will be included which features Zaeed – The Price of Revenge, The Firewalker Pack, Cerberus Assault Gear, Arc Projector heavy weapon, and Normandy Crash site mission. For Mass Effect 3, Online Pass will be included granting players access to co-op multiplayer.
I envy anybody who hasn't played these before, they're amazing.