The Avengers Take Sides for 'Captain America 3'

I don't know why they don't just re-title the third Captain America movie Avengers 3, since it looks like just about everyone will be in it and fighting each other except for Thor. Here's some concept art that was released the other day that kinda shows you who's teaming up against who. I guess they're going to explain to us how Bucky became an Avenger, right? Or did that happen already and I just forgot?