What's good enough for Harry Potter, The Hobbit, Twilight and The Hunger Games (among others) is apparently good enough for Marvel Studios, as a rumor coming out of a fan news page called the Daily Marvelite claims that the third Avengers film will an epic story split across two movies.
They admitted that all their source has told them is that the battle with Thanos will play out over two films; it could simply be that Avengers 4 will be needed to win the day, or that the third film will be subdivided.
The latter seems more likely if, as the report claims, the May 2018 and May 2019 "untitled" Marvel movies turn out to be [The Avengers 3](movie:738027) and 4.
How that would work, in terms of contracts, isn't clear; at least Robert Downey, Jr. and Chris Evans' deals will expire after one of those two films, and neither have expressed an interest in renewing their deal. That said, perhaps they'll shoot the movie all in one go and then split it into two films in post-production? Either way, it seems like they'd have to kick a little extra cash the actors' way, since knowing this far in advance that they were going to split it would seem like acting in bad faith otherwise.
The assumption on the part of most fans is that the film will be an adaptation of the Jim Starlin graphic novel The Infinity Gauntlet, which spawned a number of sequels in the Marvel Comics Universe that could serve as a thematic "second half" to the film without actually being Infinity Gauntlet, Part 2.