Neil Druckmann said in an interview that he has already outlined The Last Of Us 3's story. However, the game won't enter development anytime soon, as Naughty Dog has a few other projects in the pipeline.
https://www.notebookcheck.net/The-Last-Of-Us-3-already-has-a-story-outline-but-Naughty-Dog-isn-t-working-on-it-yet.536071.0.html
First thing done is a design document that outlines the other outlines, programming/resource allocation and basic gameplay.