I'm thinking depending on your choices in 1,2 and indeed 3, everyone you helped gets together for a big battle at the end and everyone you pissed off does not show up.
The more allies you have, you might win, the less you have, you won't win. Like your team being loyal for the ME2 ending.
I think the scene in ME2 where the Rachni Queen (if you helped her) offers you help in the end, is a clue to how the ending will play out.
With all your allies arriving, I'm thinking there will be the Alliance and Citadel fleets.. (Asari, turians and salarians and what not), the Quarians and the Geth, and then the Rachni will ball up and working together they will take on the Reaper's in one big battle. In a wonderful cinematic no doubt, while Shep runs around like a headless chicken no doubt looking for some kinda magic off button to kill all the Reapers.
Edit: I also think EDI will be a major factor. Perhaps she will go all kick ass and take over the Reaper fleet and either control them or blow them up. EDI FTW. ^_^ (I love EDI!)