Apple seeded the third developer beta of Xcode 9 on Monday, dubbed Xcode 9 beta 3 (9M174d). As with the other developer betas Apple released Monday, this release will be a mix of bug fixes and other tweaks to new features in these developer tools.
Registered developers can find the update at the Apple Developer Connection or through the Mac App Store.
Apple has a post detailing new features in Xcode 9.