This isn't Avengers 3. Did you even watch Age of Ultron? Hulk exiled himself and Thor returned to Asgard after recieving a horrific vision. He will likely be dealing with Ragnarok at the same time the events of Civil War are taking place. It's been rumored that Hulk MIGHT be in it, since Ruffalo was apparently spotted on set. However, nothing has been confirmed. He isn't listed on the cast list. My guess is he will have just a quick surprise cameo somewhere. As for Scarlett Witch, I don't know. She's in the movie, though. Also, nobody said that Captain America is going to die and Bucky will take up the mantle. Not in this movie, anyway. I know that's what happens in the comics, but it's already been said that the movie won't be following the exact storyline from the comic. They will be making changes. Not to mention, Chris Evans is still contracted for Avengers: Infinity War.