Nichole Jacque writes on the swift-dev email list:
Hello everyone. We have cut the swift-3.0-preview-1-branch. Any changes to that branch need to go through pull requests, and be approved by the release manager.
For more details on the Swift 3.0 Release Process, please check out the blog post: https://swift.org/blog/swift-3-0-release-process/
Marc points out in the comments that “The first developer preview branch swift-3.0-preview-1-branch will be created from master on May 12. It will be released 4-6 weeks later.”