Oh Marvel, you release date wizards. Right before San Diego Comic Con, the studio swooped in like vultures to claim the May 4th, 2018 release date, just after they had already announced a whole bunch of other releases. Given that Avengers opened at the beginning of May 2012, and we're getting Avengers: Age Of Ultron at the start of May 2015, it makes sense that the 2018 spot is waiting for that big third Avengers blow-out, wouldn't you say, Kevin Feige?
Speaking to Comic Book Resources, Feige unveiled the non-confirmation confirmation in regards to what exactly that release date for Avengers 3 will be.
"Well, we haven't really talked about what we'd see Avengers 3. I presume we'd like to stick to three years between Avengers movies like we did [between the first and second]. So it will probably be three years between Avengers 2 and Avengers 3."
Marvel has three release dates set up for 2018. In addition to the May 4th date, they'll be premiering an as-yet-unnamed film on July 6th, and yet another will continue the Marvel Cinematic Universe adventure on November 2nd. Those are good release dates, but they aren't first-weekend-of-May-quality release dates. So yeah, Feige might have given up the game in regards to when we'll see Avengers 3.
Phase 3 is definitely starting to take shape. We know that Ant-Man, Captain America 3, and Guardians Of The Galaxy 2, are on the way, and there's also a July 2016 Marvel movie that almost surely Doctor Strange. There are also three scheduled unnamed 2017 releases, one of which is most likely Thor 3 - unless Thor gets his ass murdered by Ultron). So that leaves two more movies. Could one be Black Widow? Captain Marvel? Black Panther? Thunderbolts? Inhumans? There are many possibilities... and perhaps even more if The Avengers 3 doesn't come out until July 2018 or November 2018.
All of this will lead to The Avengers 3, which will presumably be bringing all of the old characters as well as the new characters all together to battle the biggest of all big bads (Thanos perhaps?). Of course, there are also Avengers-level characters on the periphery of these films, like Falcon from Captain America: The Winter Soldier, or the Iron Patriot/War Machine from the Iron Man movies (the latter already being confirmed for an appearance in The Avengers: Age of Ultron. The third film could end up being a passing of the torch, or it could be something else.
We don't know much, but it seems certain that we'll be getting The Avengers 3 at some point in 2018. Or maybe Marvel thinks big, and Avengers 3 is split into two movies... or maybe even three? Much stranger things have happened.