In a move that will irritate those in the heartland dreaming of breaking into Marvel Studios but that will hearten those who think Marvel's tight reign on their productions is a good idea, the studio has hired Craig Kyle - who has been a Senior VP of production and development for Marvel Studios - to ci-write Thor 3. Christopher Yost, a comic book and TV writer who co-wrote Thor: The Dark World, is working with him. The two previously collaborated on some comics, including creating X-23, a teenage female clone of Wolverine (that one is outside of my reading experience, if we're being honest).
Kyle and Yost seem a good choice to get Thor 3 started, especially since Kyle's been a producer on the films since the beginning. And it's not like Marvel Studios scripts are free of tinkering by other writers, including 'godfather' Joss Whedon. Kyle got his start on Marvel animated projects, and I think the instincts that come from that - the ability to streamline complex continuity into something enjoyable by the masses and yet still recognizable as a Marvel story - comes into play in these movies.
Thor 3 is a big one, judging by the end of Thor: The Dark World. I keep hearing 'Ragnarok' getting thrown around, and that makes sense in a movie world where it looks like each franchise is going three entries. God knows they've gone to that well enough in the comics. In fact Thor: Ragnarok is a very good title.
What does this mean for Phase Three? We know that Captain America 3 also has writers, but we don't know where either of these films fall on the schedule. And we're currently assuming that Phase Three will continue a two movie a year schedule for Marvel, but maybe that's wrong. Waaaay back when they were making Iron Man 2 I visited the set and Kevin Feige told me he envisions Marvel having a movie out every quarter, so in a world where there are four Marvel films a year (and at this point, with the right diversification, I think they could do it), having these sequels doesn't clog up too much space. In fact act as tentpoles allowing Marvel to play around with lesser known characters like Doctor Strange, Ant-Man and Ms. Marvel (and I'm betting she'll be Ms. Marvel in the movies due to headaches when it comes to DC's Captain Marvel; while Marvel may have the name, it's too much of a hassle to bother with. It's why the aliens in The Avengers were the Chitauri. There are better things to go to the mattresses over, like Quicksilver).
Comic Con is going to be big for Marvel this year; they're going to almost certainly have The Avengers: Age of Ultron footage to show (and maybe they'll reveal some new characters, if they don't get spoiled by then), and I think they'll fully lay out Phase Three. We'll have release dates and titles for all the films, including possibly films we haven't even guessed at yet.