Today, Apple began seeding both developers and public beta testers with the third beta of macOS Sierra 10.12.6 for debugging on the Mac. This comes two weeks since the second build landed.
If your Mac is registered to receive preview builds, head over to the “Updates” tab in the App Store and check for updates. There, you’ll find the new build ready to install. Just click the “Update” button next to the macOS Sierra 10.12.6 Beta 3 listing and restart your Mac. The process may take a little while, so be patient and don’t turn off your Mac manually if you don’t have to.
So far, only bug fixes and security improvements have been found within this build of macOS 10.12.6 which makes sense since this is a cumulative update. If you notice something we may have not, forward your sightings to firstname.lastname@example.org and @MBEDDEDMaximum on Twitter. It’s worth noting that the second beta of macOS High Sierra wasn’t released today as we likely have to wait another week or two for the public release of the build to hit devices.