This was a part of an April Fools' Joke, even though it's been over a week since. There will be a Nico Live event with huge announcements on April 10, 2014. Whether there will be a third season or not will be confirmed then. There was no anime season 3 announcements during the Nico Live (both Index and Railgun). I apologize for having been trolled.
According to a magazine scan published via a tweet, Toaru Kagaku no Railgun
will receive a 3rd season. The staff that was revealed on the scan remains the same. More information will come at a later time.
Source: Tweet via Yaraon