Marvel Studios unveiled the movie release schedule for Phase 3 of its Cinematic Universe today, at a special event where various journalists, bloggers, and members of the press were in attendance. Among the films that received official release dates were the next Thor installment, which is now officially titled Thor: Ragnarok.
The third Thor solo film will see Chris Hemsworth reprise his role as Thor Odinson, in a story/script that is being written by Craig Kyle (the 2007 Doctor Strange animated feature, Thor: Tales of Asgard) and Christopher Yost (Thor: The Dark World). We've yet to hear about additional supporting cast members, though Tom Hiddleston is expected to reprise as Loki - given the state of things regarding Thor's trickster sibling at the conclusion of The Dark World - and Anthony Hopkins should likewise be back as Odin (in some form, anyway).
Marvel Studios' head Kevin Feige, when he spoke at the Marvel event, explained to the crowd that "Ragnarok means "The end of all things'" - and thus, the film will see Thor and his comrades engaged in an apocalyptic battle for Asgard.
As far back as our visit to the Thor: The Dark World set, though, we had begun hearing (including from such people as Kyle) things that pointed to Ragnarok being depicted in the third Thor movie. It's believed that the event, in cinematic form, could see Thor and his allies take on the Fire Demons of Muspelheim, including Thor's great enemy Surtur. The latter is a villain that Hemsworth has voiced an interest in getting to fight onscreen, and the character was included in the Thor: God of Thunder video game (see below).
A director has yet to be announced for Ragnarok, though the film is now scheduled to open in theaters on July 28th, 2017. There's still plenty of time for a helmsman to be found, as shooting on the next Thor installment won't get started until near mid-2016, after Hemsworth has fulfilled his obligation to star in another upcoming franchise title - the Snow White and the Huntsman prequel/spinoff by writer/director Frank Darabont, titled The Huntsman. (As Hemsworth once put it, he has "a lot of time" before the cameras start rolling on Ragnarok.)
You may recall that was the July 2017 date that was previously reserved by Marvel Studios for director James Gunn's Guardians of the Galaxy 2, but Gunn's space adventure sequel has now been officially moved forward to May 5th, 2017. Meanwhile, rounding out 2017 for the studio will be Black Panther on November 3rd, 2017, with Chadwick Boseman (42, Get on Up) playing that film's namesake, after he makes his onscreen debut as the character in 2016's Captain America: Civil War (previously known as simply Captain America 3).
As for what the future holds for Hemsworth as Thor (and the character in the Marvel Cinematic Universe), well, that is an interesting question. As you most likely know or have heard, the God of Thunder's hammer Mjolnir in now being wielded by a woman in comic book form, and it's possible that the MCU will follow in the footsteps of the Avengers Now! comic book reboot, in that respect. WHEN and/or IF that will happen, however, are questions that may not be answered for a while...
The Avengers: Age of Ultron releases in theaters on May 1, 2015, followed by Ant-Man on July 17, 2015, Captain America: Civil War on May 6, 2016, Doctor Strange on November 4, 2016, Guardians of the Galaxy 2 on May 5, 2017, Thor: Ragnarok on July 28, 2017, Black Panther on November 3, 2017, Captain Marvel on July 26, 2018, Inhumans on November 2, 2018, Avengers: Infinity War - Part 1 on May 4, 2018 and Avengers: Infinity War - Part 2 on May 3, 2019.