The new macOS High Sierra 10.13.3 beta can be downloaded from the Apple Developer Center or through the Software Update mechanism in the Mac App Store with the proper profile installed.
It's not yet clear what improvements the macOS High Sierra 10.13.3 update will bring, but it's likely to include bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.2.
It does offer additional fixes for the Spectre and Meltdown vulnerabilities that were discovered and publicized in early January and fixed initially in macOS High Sierra 10.13.2.
The update also fixes a bug that allows the App Store menu in the System Preferences to be unlocked with any password.
The previous macOS High Sierra 10.13.2 update focused solely on security fixes and performance improvements, with no new features introduced, and a supplemental update introduced a fix for the Spectre vulnerability.
Update: Apple appears to have temporarily pulled the macOS High Sierra 10.13.3 update, and it is no longer available for download from the developer site.
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