User blog comment:Sim0n2170/Attack on Titan Season 2 announced for SPRING 2017/@comment-5774380-20160703133127/@comment-27321453-20160703185757

Since the manga is ending in 2018 I doubt season 3 would be the same year. It could be late  2019 or early 2020 or they might just decide to work on other projects and delay it to 2021.