Today, Apple began seeding testers with the third build of macOS Sierra 10.12.5 for debugging before an initial rollout to the public. The beta comes a week after build two was released.
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 10.12.5 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 10.12.5 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.