Updated: Today at the Dengeki Bunko Festival it has been confirmed that Index 3 will be happening in 2018. However there was also a hint Index 3 may not be the final project, could Railgun 3 be making an appearance or an adaptation of A Certain Scientific Accelerator happen as well?
It has been reported that a teaser for A Certain Magical Index III was mistakenly posted to Animax’s webpage. While normally this wouldn’t be cause for too much concern the timing of the “mistake” is a little too much to ignore. As the Dengeki Bunko Aki no Saiten 2017 (Dengeki Autumn Festival) is set to launch on October 1st, Dengeki Bunko being the publishers of the Index Light Novel.
According to ANN the announcement read:
A Certain Magical Index Season 3's Broadcast Green-Lit!
A Certain Magical Index, captivating many fans since season 1 began airing in 2008 and season 2 in 2010. Over six years later, the broadcast of season 3 has been green-lit! About a decade after season 1, what kind of story will turn out?
To myself this just seems to polished to be a mistake. Regardless we should know for sure on October 1st. Until then file this under rumor.