I think it's going to do very well since Iron Man is apparently a big part of it. If it was just a straight sequel, then I think you'd have been right. But Iron Man is extremely popular, and Iron Man 3 showed that. Now, if you add the fact that Spider-Man is going to play some kind of role, then I think it stands a very good chance of passing Iron Man 3 numbers as it stands. I think it will anyway.
It's not really Captain America 3. It's Captain America, with Iron Man in a significant role, and a bunch of other superheroes, including Spider-Man's introduction to the MCU. That last part is also important because it's the one factor that makes it very interesting. We don't really know how big of a role he has, or whether it's just a cameo (or I don't but I don't exactly keep up with the news). I honestly thought BvS was going to comfortably be the highest grossing movie next year, but since this is basically Avengers 3, and with Spider-Man, it makes it a little more difficult to call. I still think BvS will do better, but I think they're both going to fall in a similar range. Spider-Man's recent movies may not have shown any growth, but he's still a huge attraction, and his introduction into the MCU should garner some major attention. Depending on how they market his involvement, it can be significant.
Folks shouldn't really sleep on Iron Man either. That move, I think, ensures this movie grosses beyond a billion. Between a billion and Iron Man 3 numbers, there isn't a huge difference with the way certain international markets have grown. Iron Man 3 was an Iron Man movie after all, and Iron Man is in this. I don't think a billion is in doubt now thanks to him. It's simply a case of how far Spider-Man could possibly take them beyond that Iron Man factor.
This movie feels like it could be a bigger deal than Avengers 2 to me, especially if Spider-Man ends up being a notable part of the marketing campaign. Iron Man is there, and apparently half of the other jobbers in the team are too. When they start plastering Iron Man, and Spider-Man everywhere, it's not going to really matter if the movie is called Captain America 3 or whatever. I'm expecting numbers similar to Avengers. I can't see how it doesn't, at least, fall in that range. $1-2-1.5 billion should be happening in my eyes. It feels like a pretty safe call even without really looking into it that much.