Microsoft has announced availability for AX 2012 R3 Cumulative Update 12 recently, and just became available for download today. You may find the announcement with the What’s new links here:

https://blogs.technet.microsoft.com/dynamicsaxse/2016/11/21/announcing-cumulative-update-12-for-microsoft-dynamics-ax-2012-r3/

According the the Build numbers website the kernel has version 6.3.5000.138, however a newer build is already available:

https://blogs.msdn.microsoft.com/axsupport/2012/03/29/overview-of-microsoft-dynamics-ax-build-numbers/