Borderlands 3 isn't currently in development
You'd think that with the success of Borderlands 2, Gearbox would already be hard at work on Borderlands 3, but that isn't the case. "I'm not going to fuck around with you like Valve does with Half-Life 3," Gearbox CEO Randy Pitchford told Polygon. "We know we want it and we know it should exist, but we don't know what it is yet."
You have to appreciate the honesty. It's not like we won't get another Borderlands—it's just too early at this stage for Gearbox to talk about it in any concrete way. Loosely speaking, Pitchford added that he knows that the game should be "next-gen" and "probably crazy multi-platform." He also said that it should be bigger and better than Borderlands 2, and follow up Borderlands 2's story. That's certainly what we're expecting given the future beyond planet Pandora that's hinted at in the end of Borderlands 2.
Pitchford did imply, however, that more Borderlands things are afoot at Gearbox, even if the developer hasn't begun work on a fully fledged sequel. "We are doing things in Borderlands that we'll announce soon, that are good, and that I think people will be really excited about if you love the franchise," he said. There's also Telltale Games' Tales from the Borderlands, expected to release in 2014.
Aside from Borderlands, Pitchford said that Gearbox is working on two, yet-to-be-revealed games, unrelated to existing franchises. And this on top of its Furious Four, another Brothers in Arms game, remastered editions of both Homeworlds and Homeworld: Shipbreakers.