I'm gonna try to describe the storyline based on what I understand. The 3rd installment starts when the Reapers attack Earth. With Earth's higher councils all dead, Commander Shepard has to travel to the Citadel and talk to the councillor in getting their support to fight the Reapers. A further complication to the situation is the revelation that Cerberus (which saved Shepard in the last installment) are working with the Reapers as well. Along the way, Shepard must gain the trusts of other species by helping them save their people in return for their fleets to join forces. Whether Shepard succeeds in saving Earth is all up to you as every decision you make can have a different outcome.
I have to say this game is the best of the 3. It improved so much in terms of storyline, gameplay and whatever you can think of. In terms of storyline, it's defnitely more engaging and you just feel for the characters and what they go through. I wish this gets turned into a movie 'cuz I can definitely see this being epic of Star Wars proportion. The graphics have definitely been improved with more details in the look of the characters and environment. The different settings and details of the missions gives you a whole new experience that doesn't feel recycled like previous games. Some side missions are more elaborate than ever. Collecting resources now can also help determine the outcome of the game as you are looking to expand your war fleet. Gameplay has certainly improved as the combats are now tougher than ever and I don't feel the 1/10th second of lag I felt playing previous games. The toughest "mutant" I would say is the Banshee. The first time it came out, I was freakin' scared and it moves faster than you can roll. Luckily enough I learned the ropes of escaping its attacks. With the Reaper conquering some of the systems, galaxy exploration is now tougher as well.
New feature added to this game is the Kinect voice recognition. At first I thought it wasn't gonna be supported over in Asia but it actually is. Just make sure you're accent is either American or British. You can have the option of choosing dialogues by just saying it out towards Kinect. This also works for actions like opening a door, picking up weapons etc. Another feature you can go for is to skip any dialogue choices. It will automatically select the dialogues for you so that it looks like a cut scene and you can just focus on the combat. This is good for those who aren't really into RPGs but wants to experience the awesome shooter element of the game. Weapons can now be upgraded with mods (Can't remember if it was in the previous games). Only 1 issue I found with game and it is the cover system. I think it's a glitch but there are times when I take cover and the character just automatically slides along the cover, sometimes turning to another side of the cover if it has more than 1 side. This sometimes exposes the character to enemy fire. Some people were unhappy with the ending, and I get them. It's very open ended yet they state that it's the end for the trilogy. You can always download the demo first to try it out and see for yourself.
My conclusion: This is one of the most exciting games I've ever played. This 3rd installment has improved on every shortcomings from the first 2 games to make it even more engaging and interactive for both RPG and shooter fans. Everything about it is epic. Sad to see it all end though :(