Anonthenine’s post was made in response to another user who speculated that Bungie would have a “solid base running version of Destiny 3 by now”, in which they replied by saying, “Nope. The development its starting right now. Chris Barrett game director. Luke Smith still there in a big role.” No specifics were given except to say that the game is in the works.
That being said, it is unclear as to when the game will be released, but considering that Destiny 2 is just over a year old, we doubt that it will be released anytime soon. Also assuming that Bungie and Activision are aiming for the same time in between releases, chances are it might only be 2020 or 2021 that we will actually see the game, although it is possible that maybe they might announce it beforehand.
Needless to say that this should be taken with a grain of salt for now, but Destiny 3’s release should be interesting because there are talks that Sony’s PS5 could be launched late 2019 or 2020, so it remains to be seen if Destiny 3 could be made with next-gen consoles in mind.