According to Double Fine boss Tim Schafer, don't expect a sequel to Psychonauts 3 to be the next project. Instead, the studio will focus on creating something completely new.
In an interview with IGN, Schafer explained that Psychonauts 2 was "the first truly substantial sequel to a big story" the studio has ever made. Because of this, Double Fine's boss added:
I think this is what motivates me to create new, original things in the next stage. Coming up with new ideas is what I'm doing now.
However, Schafer did not rule out that someday the studio would return to Raz and his buddies, adding "never say never."
It is also known that after the completion of the development of Psychonauts 2, Double Fine took up several projects. The studio split into several teams and started working on different projects that gamers should like. It is not yet known what projects we are talking about, but it is almost certainly known that Psychonauts 3 is not among them.