We've known about a Borderlands 3 for a while. It was announced that there would be another in the series last year.
However, that has been reaffirmed by Gearbox head Randy Pitchford during a panel at PAX East. During the discussion, Pitchford said quite plainly:
Obviously, there is going to be another Borderlands.
He did however say that it might not be called Borderlands 3 when it does land.
It was also said that Mikey Neumann, who wrote the first Borderlands, and of Movies with Mikey fame would be back on the writing duties for the outing.
While I'm not a huge Borderlands guy, I know the fans for the series are rabid, so this will surely please them.