Quoting a post on Reddit which explains certain scenes:

Will it be faithful to the source material?
The author of the manga, Isayama Hajime, revealed in an interview included in the Season 2 Blu-ray boxset that he was “disappointed” with his work on Volumes 13-16 (which will be covered in Season 3). He says: “Hopefully, now that Season 3 is being made, I’ll have the chance to wipe away that regret and replace it with something I’m proud of.” This indicates that there will be some differences between the manga version and the animated version, but we don’t know yet exactly what those changes will be like.

Based on the Season 3 premiere event, it is evident that the pacing and order of certain scenes is being shifted around. Some events were shortened, some were apparently skipped over, and some were moved to different points of time with different contexts. Until the rest of Season 3 airs, it will be difficult to tell how extensive the changes will be. However, it seems as if the core story will remain identical to the manga, and will simply be executed differently.