According to Collider, casting notices have been posted in and around Atlanta calling for extras to populate Cap 3. Along with it comes the following storyline for the film,
Following the events of Age of Ultron, the collective governments of the world pass an act designed to regulate all superhuman activity. This polarizes opinion amongst the Avengers, causing two factions to side with Iron Man or Captain America, which causes an epic battle between former allies.
What in Age of Ultron could possibly warrant the world’s governments to pass such a law?
Though we’re still figuring out the details of the Civil War adaptation, it seems more than likely that Scarlet Witch and Quicksilver have a lot to do with it. As we learned through a prequel comic for Age of Ultron, Wanda and Pietro Maximoff agreed to become the subjects of Baron von Strucker’s experiments for Hydra. After he manipulated the stone in Loki’s scepter — which is an Infinity Stone — the twins emerged as the only survivors to this harsh process, and with superpowers, no less. Agents of S.H.I.E.L.D. showed us how the government is already watching a number of beings with superhuman abilities, but these two are coming from left field with powers the Avengers are ill-prepared to face. In addition, we’ll see Ultron create another future superhero from scratch: The Vision. It seems that the world’s leaders create this act as a means of further eliminating this element of surprise.
We already know which side Cap will take. In an extended clip from Age of Ultron, we see how he’s already philosophically sparring with Iron Man over what it really means to try to "end the war before it starts."
Which side will the other heroes take: Cap’s or Stark’s? Civil War will also feature Black Widow (Scarlett Johansson), Hawkeye (Jeremy Renner), Falcon (Anthony Mackie), Black Panther (Chadwick Boseman) and The Winter Soldier (Sebastian Stan). In addition, we expect Marvel and Sony’s new Spider-Man will make his first appearance. Filming will begin in the next couple months for a release date of May 6, 2016.