While most of us probably expected Naughty Dog to already be working on The Last of Us: Part III, something that also was rumoured last year, it now seems like the development is fairly far along.
According to ViewerAnon on Twitter, there is "major filming is happening this year" for the game, something that probably means motion capture work (if true), which usually is done when a project is towards the end of full production. The same account also claims that "Ellie is at least as important in LAST OF US PART III as she was in PART II", which frankly isn't that surprising and in line with what most of us thought anyway.
While we're not really familiar with the source, it's a fairly big Twitter account that has been around for quite some time, but we would recommend you to take all this with a generous amount of salt.