In the end of Resistance 2, you blow up the main Chimera (bad guy) ship and all the other ships explode, killing all their fleet but opening some wormhole that makes it seem like you won, but when you get to Resistance 3, it's set years into the future, and it turns out humanity has lost. Civilization lies in ruins, and the Chimera are getting all set to take over Earth (also, they have underground structures that have never been discovered before their arrival. Sound familiar?), but you, a lone hero, have to go to New York City and blow up this wormhole-stabilizer-thing, and when you do, you save the day.
Sound familiar? I don't think I need to explain.
I was hoping that Resistance 3 would be more of an all-out war against the Chimera. Taking place all over the world with great variety, story, etc. However, it wasn't, and I was disappointed, but with what Insomniac Games set up, they did a very good job. I had the same hopes for Crysis 3, but looks like I'm disappointed again. Only difference is that this game is being released two years after 2, whereas Resistance 3 was released three years afterwards, so there's more of a chance this will be screwed up.
I have faith in Crytek that this game will be awesome. I don't really like the setting, and I don't know if it's supposed to be the end of a trilogy, but I eagerly await it.