A site known as "The Daily Marvelite," which I've never heard of, claims that Avengers 3 is so massive it has to be split into two films. There's one immediate problem with this rumor - they haven't even finished editing Avengers: Age of Ultron yet, so I kind of doubt Marvel has anything set in stone for the next film, especially with Phase Three about to introduce a ton of new characters. Marvel will want to see how audiences react to Ant-Man and Doctor Strange and Captain Marvel before they make big decisions about what's next. But there's another major problem with this rumor: the Salkind Clause.
You know Ilya Salkind as the man who produced Superman: The Movie, but his biggest legacy may be with the Screen Actor's Guild. Salkind produced The Three Musketeers, directed by Richard Lester and starring a killer cast that included Oliver Reed, Charlton Heston, Raquel Welch and Faye Dunaway. The script was big - it was considered to possibly be a four hour movie - but during post-production it was decided to cut the film in half, releasing The Four Musketeers the very next year. This incensed the actors, who found themselves starring in two movies but only being paid for one, and lawsuits were filed. Eventually the Screen Actors Guild included new language in the standard actor contract, known as the Salkind Clause, which says the actors must know how many movies they're making when they sign up - ie, you can't just use all their extra footage to make another movie without paying them for another movie.
The Salkind Clause becomes important because Avengers 3 will mark the contractual end of the line for both Robert Downey Jr and Chris Evans; Evans will have finished his sixth Marvel film, and Downey renegotiated his contract to carry him through Avengers 3. Marvel cannot simply shoot a lot with these guys and then split the movie in two - they would need to renegotiate their contracts to split Avengers 3 in half, and that would mean that Evans and Downey would straight up have the studio over the barrel.
Of course there's precedent for such renegotiations. Warner Bros recently had to do it with the cast of The Hobbit films when they went from two movies to three. It could be done, if Marvel decided to stop being cheap and to start paying out big time - but Evans would certainly demand a payday equal to Downey, who netted over $50 million for The Avengers.
But even if Marvel is willing to deal with the Salkind Clause, the 'splitting in two' thing makes no creative sense. That's just two movies, not one movie split in two. While the next Avengers will certainly have a title from a comic book, it's unlikely to be a straight adaptation, as none of the Marvel films have yet been straight retellings of comic stories. This isn't like Harry Potter and the Deathly Hallows, where they took the existing material and used it for two films - just tell your story in one film!
All of this said, anything is possible. A big full-universe crossover movie post-Avengers 3 could be the film that beats Avatar at the all-time box office. It could once again redefine the franchise movie model. If Marvel goes that way, all the hardball renegotiations might be worth it.