The most recent scoop/rumor concerning Avengers 3 is that the movie will be split in 2, with release dates on the May 4, 2018 and May 3, 2019 (usually, early May releases are reserved to Avengers).
Another hint that this might actually happen comes from Groot himself, Vin Diesel hinting at a merging of the brands, which also means that eventually the Avengers and the Guardians of the Galaxy will share some screen time.
In addition to that, battling the Mad Titan Thanos would most certainly deserve such a move, a move most big films have made in the recent years.
What do you think? Will this happen?