Jeremy here...
It's as official as it can get without a Marvel Studios press release: according to Badass Digest, Joe and Anthony Russo have closed their deal to direct both chapters of THE AVENGERS: INFINITY WAR.
This was expected, but you never know how contract negotiations will go (e.g. Adam McKay entering talks to direct ANT-MAN last year before backing out twelve hours later). The Russos are preparing to shoot CAPTAIN AMERICA: CIVIL WAR, and have been lobbying to take on the third AVENGERS flick. Fortunately, Marvel couldn't be happier with their work, so working out a deal was strictly a formality. Now the studio just has to do likewise with screenwriters Christopher Markus and Stephen McFeely (who've written all three CAPTAIN AMERICAs), and they're locked in through 2019!
CAPTAIN AMERICA: CIVIL WAR is due out May 6, 2016. THE AVENGERS: INFINITY WAR - PART ONE hits on May 4, 2018, while PART TWO arrives on May 3, 2019. Both AVENGERS movies will be shot back-to-back.