Out in December in Japan
Japan already got Attack on Titan: The Last Wings of Humanity some time ago on the 3DS, but now developer Spike Chunsoft is releasing a “Chain” expansion later this year. It’ll have online multiplayer, as well as new weapons, missions, and zones to explore. It’s set to release in December, and offers an upgrade promotion if you own the original.
I wouldn’t rule out an Attack on Titan 3DS localization at some point in the future, given how big the show is in other regions at the moment.