In Mass Effect 3 for the PC, PS3, and Xbox 360, seeing the different endings depends on a few different things.
The first is by choosing to destroy or save the Collectors Base in Mass Effect 2. This will alter the outcomes at the end of the game quite a bit based on this choice alone. If you didn’t play or didn’t finish Mass Effect 2, it will default to destroying the based when you start Mass Effect 3.
The second way to see the different endings to the game is by gaining a certain number of effective military strength.
Military strength is raised several different ways:
In single player, military strength raised primarily by gaining war assets. There is a limited number of war assets in the game, but there is supposed to be enough in the game to see the best ending. You’re gonna need 8000 military strength without playing online though, so it’s gonna be tough.
Another way to raise your military strength is by playing in the Galaxy at War multiplayer games. Playing in Galaxy at War increases your galactic readiness rating, which raises your effective military strength. This will allow you to see the better endings without having to collect as many war assets in the single player mode.
There are also some iPhone and iPad apps that will help boost your galactic readiness rating. One is called Data Pad, and the other is called Infiltrator. Obviously you’ll need to sync your EA account in order to apply your ratings.
Important note: If you don’t play any multiplayer games, then your readiness rating stays at 50%, which is how much of your military rating is used to calculate your “effective military rating”. For example, if you have 5000 military rating, but your readiness rating is only 50%, this reduces your effective military rating down to 2500. Long story short, you probably want to play online some in order to see the best endings effectively.
Check out my other guide for the complete list of endings to Mass Effect 3 and what military strength you need to see them.