Everyone expected a third Avengers film a few years from now, but Marvel has a surprise for you: you'll be getting two of them. The third Avengers film is called Avengers: Infinity War, and it's being broken into two parts: its first part is being released on May 4th, 2018, and its second part is being released on May 3rd, 2019.
A huge battle against Thanos
The split is perhaps an obvious business decision. Ever since the final Harry Potter film, young-adult franchises have made a habit of splitting up their final films to drag viewers back year after year. For Marvel, this may also be a way of bringing its most beloved characters back one last time — especially given that many actors' contracts could be up after these final films.
Infinity War is also likely to be a huge story. This should finally conclude Thanos' war against basically all life — an ongoing plot that's been threaded throughout Marvel's cinematic world. Given that Marvel's entire film universe is, in some sense, one giant story, it makes enough sense that it would want to take plenty of time to tell the capstone entry.
Marvel is supposed to have all of its films mapped out through 2028, and it's also likely that Infinity War will play a big role in setting up what's to follow. That's going to be true of the upcoming second Avengers film too, with Marvel Studios president Kevin Feige speaking to Buzzfeed about this recently with a revealing, but potentially spoiler-filled statement: that the Avengers' roster would be different by the end of Ultron.
Marvel announced the titles and release dates of all of its movies up through these final Avengers today, all of which you can see here.