Toaru Majutsu no Index III [Special Stage] at Dengeki Bunko Fall Festival 2018 

Miki announced the A Certain Scientific Accelerator (Toaru Majutsu no Index Gaiden: Toaru Kagaku no Accelerator) manga is getting a television anime adaptation in 2019 (13:15)
Later (20:22) Satomi Arai (Shirai Kuroko VA) surprises fans revealed that the A Certain Scientific Railgun anime series is ALSO getting a third anime season.
Kazuma Miki stated during the event that while the second season of the anime had the "S" subtitle on it, he does not yet know what the third season will be called.