Apple has released MacOS Mojave 10.14.3 beta 3 for users enrolled in the beta testing programs for Mac system software. A developer build is usually available first, soon followed by the same build for public beta testers.
Separately, Apple also released iOS 12.1.3 beta 3 along with new betas for tvOS and watchOS.
The new beta build of MacOS Mojave 10.14.3 beta is 18D38a and can be downloaded from the Software Update section of System Preferences.
It’s not entirely clear what MacOS Mojave 10.14.3 beta focuses on, but presumably the update aims to resolve lingering bugs and other issues in MacOS Mojave.
Typically Apple goes through several beta builds before releasing a final version of system software to the general public, so it’s reasonable to expect the final build of 10.14.3 is still some time away.
The most recent stable build of MacOS Mojave is currently MacOS 10.14.2, whereas iOS 12.1.2 is the most recently available stable build of iOS.