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Following a break for the holidays, Apple has issued a new batch of developer betas for testing, with third beta builds of iOS 12.1.3, macOS 10.14.3, watchOS 5.1.3, and tvOS 12.1.2 now available for download
The new builds can be picked up by developers enrolled in the beta as an over-the-air update to their devices, or through the Apple Developer Center. The public beta counterparts usually arrive a short time after the developer versions, and can be acquired from the Apple Beta Software Program. Public beta versions of iOS 12.1.3 and tvOS 12.1.2 were pushed out shortly after today's developer releases.
The third iOS 12.1.3 beta is build number 16D5037a, replacing the previous build 16D5032a. macOS receives a build number of 18D38a, while Apple's third watchOS 5.1.3 beta, build 16S5535a, takes over from the second beta, build 16S5531a. Lastly, the newest build for tvOS 12.1.2 is 16K5534a, incrementing from 16K5532a.
The betas indicate these to largely consist of bug fixes and performance improvements, rather than updates that introduce new features to each respective operating system.
AppleInsider, and Apple itself, strongly suggest users avoid installing betas on to "mission-critical" or primary devices, due to the potential for data loss or other issues. It is instead recommended to install betas onto secondary or non-essential devices, and to ensure there are sufficient backups of important data beforehand.
Updated with information about macOS 10.14.3 and Apple's public beta program.