It seems to be confirmed that Disney XD's Avengers: Earth's Mightiest Heroes has not been renewed with a Season 3.
Back in early May, a rumor hit the net that Avengers: Earth's Mightiest Heroes was to be canceled to make way for a new Avengers animated show that is to be part of a shared Marvel cartoon universe along side the new Ultimate Spider-Man.
Now in a reported press release, we have confirmation that the rumor is true (at least the cancelation part), and the new cartoon will be called "Avengers Assemble," which is the same title of Marvel's newest Avengers comic book.
Though I haven't found the press release anywhere online, MarvelTVNews has the following information, in addition to an image. They also stated something about another rumor that says Avengers Assemble will be on Cartoon Network, which doesn't make much sense seeing how Disney owns Marvel and Disney XD — but, I believe, Cartoon Network is part of the lower tier cable package, while Disney XD is part of the upper tier/digital packages, which would mean Cartoon Network is in more homes.
Regardless, here's what they have:
The Avengers: Earth’s Mightiest Heroes” won’t be renewed for the 3rd Season. Marvel Animation will present their newly developed series “Marvel’s Avengers Assemble” in 2013.