The 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 third update to macOS High Sierra 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.
No major outward-facing changes were discovered in the first two betas of macOS High Sierra 10.13.3, but we'll update this post should new features be found in the third beta.
The previous macOS High Sierra 10.13.2 update focused solely on security fixes and performance improvements, with no new features introduced.
Update: Apple has also made a public beta available for public beta testers.