As of 1-29-14, Marvel hasn't revealed any details for 'Thor 3' and the opening date is unknown. Even though the opening date is a mystery right now, it's a safe bet to expect 'Thor 3' sometime in 2016.
Last year, Chris Hemsworth and director Alan Taylor talked about what might be in Thor's future.
Need more info? Check Marvel's press release via the Tweet below.
#Marvel’s #ThorDarkWorld EP Craig Kyle transitions to screenwriting with next #Thor: http://t.co/R4YwomOrWg
— Marvel Entertainment (@Marvel) January 30, 2014