I think we can agree that the final showdown will be Shep and co. staring down the barrel of a massive Reaper fleet. All hope is lost, the galaxy will fall and there's no way you can stop it. That is, until all your buddies made throughout the series (the Geth, the Rachni, the council, Hackett and the Alliance, the Migrant Fleet etc.) ride in all F*CK YEAH Han Solo style and tip the odds in your favour.
I really do hope your actions from all three games have some effect on the outcome of the trilogy. Sure, the game has to work as a stand alone piece so you should be able to "win" and save the galaxy, but it shouldn't be the only game you need to play to get the best ending. It felt that, in ME2, while playing ME1 gave you a lot of extra stuff, it didn't have that much impact on the story and how the game played out. To be honest, if you're buying ME3 without playing either of the prequels you don't deserve the best ending
There better be a "what the hell have you been doing
for three games?!" ending too, where everything gets f*cked over in the worst possible way.