Funimation will actually be doing a Simuldub project for Attack on Titan in English, so they have an idea when WiT Studio will release this new project.
It’s should be noted that it’s Funimation which made the announcement, and not WiT Studio, which animates Attack on Titan.
During the final episode of Attack on Titan season 2, the anime ended with an announcement that Season 3 will premiere in 2018.
Now, during the Anime Expo 2017 event in Los Angeles, California, Funimation announced that Season 3 will be premiering in Spring 2018, albeit “tentatively”.