Marvel Announces Avengers: Infinity War
As part of the massive Phase 3 reveal today, Marvel announced that the third Avengers would be called Avengers: Infinity War. It's technically the third and fourth Avengers films, as it will be split into two parts.
The films will presumably be based on the Infinity War limited series, a six-issue sequel to the Infinity Gauntlet crossover. (We probably should have known that Marvel would resuscitate the comic that would serve as inspiration for the Avengers 3 film.) In this series, Adam Warlock's villainous alter ego Magus creates an army of doppelgängers, or evil mirror images of Earth's superheroes.
Thanos also plays a large role in the comics, which may mean that there will be crossover between the Avengers series and Guardians of the Galaxy. This theory is supported by a picture released from the Marvel event:
In this picture, Thanos has all six of the Infinity Gems. In the comics, Thanos combines these gems to create the Infinity Gauntlet, and then used the Gauntlet to wipe out half of the population of the universe. So it might be safe to say that Avengers: Age of Infinity will have a fairly dark tone, following the trend of Captain America: The Winter Soldier, Avengers: Age of Ultron (judging from the trailer), and likely Captain America: Civil War.
Avengers: Infinity War part 1 will be released on May 4, 2018, while part 2 will be released on May 3, 2019.