I thiiiiiink Borderlands 3 had already been confirmed in a bunch of "yeah, in the future" and "we want to" ways but this time there was mention of actual project staff by Gearbox's CEO Randy Pitchford and that there will likely be little references to the game in Battleborn DLC so it's more concrete as information goes…
Eurogamer attended the panel which featured the announcement at PAX East and they explain that Scott Kester has been named as art director (he's currently in that same role on Battleborn and was one of the main concept artists on Borderlands and Borderlands 2) and Mikey Neumann will be writing (he was one of the writers on the original Borderlands and the voice of Scooter).
With regards to the Battleborn stuff (oh God, I am going to spend half my life checking I've not mixed up Battleborn and Borderlands in articles, aren't I?) Pitchford added that Battleborn's team will be working "to sneak Borderlands eggs in DLC". More specifically, as per the panel report, the main Battleborn game has Borderlands Easter eggs in it anyway referencing the existing games in the franchise but there's now the possibility that future games will be referenced in the Battleborn DLC.
I assume this means Easter eggs like little in-game clues and surprises but maybe they will be literal eggs as well. Like, you buy some Battleborn DLC and your computer lays an egg with Handsome Jack's face on it or something? Or maybe you have to look after the egg like when you're at school and they make you build a go-kart for an egg race but the egg can't get broken in transit otherwise you lose and you aren't allowed to boil the egg either but if you keep your egg safe then you get a copy of Battlelands? Borderborn? Bottleband? Brittlebeak? Bristleback? Bartleby? free of charge.
The Gearbox marketing department's loss is RPS's gain.
*The title isn't confirmed, by the way. I'm just referring to it that way because you know what I mean when I say it. More accurately you could call it "the game which you could feasibly name Borderlands 3 as it's in development for the franchise by the studio which made Borderlands and Borderlands 2 and assisted on Borderlands: The Pre-Sequel but which might end up being Borderlands 4 or Borderlands 3.5 or maybe even Borderlands: The Fight For Mango Mountain if they've gained access to my secret screenplay ideas vault". You can see why I went with Borderlands 3.