Following up the release of the third beta of iOS 11.4, Apple has also released a third, and likely final, beta build of MacOS 10.13.5 to beta testers. The Apple Beta Program is open to anyone with an eligible device and you can sign up if you want at this site. Remember, it is beta software so you may find the odd bug or behavior as final testing is done.
Much like the iOS 3rd beta, the MacOS 10.13.5 3rd beta more or less is a polishing and bug crushing exercise. There aren’t any new features of note other than those which are already known about, namely Messages in iCloud and AirPlay 2.
The question on everyone’s mind of course is when will this next build of High Sierra be released. Apple, like always, isn’t being forthcoming with a release date but given this is the third beta and that WWDC is coming up early in June, it is likely we will see it released there or around that event. That assumes of course no major issues are found with the current beta build being offered.
Also keep in mind that it is very likely that the next major version of MacOS, 10.14, will be announced at WWDC. If you recall, High Sierra was announced at the developer event last year and released in September 2017.