The chunk of code that will become version 3.0 can now be found in the Git repository at GitHub. Please note that it is (at the time of writing) very early in development and that things may not work yet or even be present at all. The current state of the code is primarily intended to foster discussion.
Development of version 3.0 is coordinated via the glfw-dev mailing list.
There is a preliminary roadmap for 3.0 on our wiki.