Apple seems to be taking its time getting things right with Logic 10.6 before moving on to the next greatest and latest new features. This third sub-release of Logic Pro 10.6 further refines the overall stability. 

I, for one, have been experiencing the problem of intermittent crashes loading projects using pace/iLok copy protection. I will be happy to see that problem in the rearview mirror now. Further refinements to delay compensation for busses containing plug-ins that are side-chained to tracks is a welcome enhancement. Here is the complete list of new improvements in Logic Pro 10.6.3:

New Features/Enhancements

The Plug-in Manager now offers a way to reset Audio Unit caches and perform a rescan of all installed Audio Unit plug-ins.

Stability/Reliability

Fixes an issue where Logic Pro might unexpectedly quit when loading projects with plug-ins that use PACE/iLok copy protection.

Resolves an issue where Logic Pro quits unexpectedly when Option-dragging a marquee selection within a folder.

Sampler/Quick Sampler

Key label sets can now be saved in Sampler.

Mixer

Resolves delay compensation issues for plug-ins that require look-ahead, and for busses containing plug-ins that are side-chained to tracks.

Duplicated tracks based on tracks that have a user-defined default patch loaded are now named correctly.

Audio tracks created by bouncing software instrument tracks in place are now named correctly.

ARA

ARA edits are now properly retained for saved projects containing muted ARA regions.

Editing

Fixes an issue where changing Smart Quantize parameters could reset prior edits to note velocities.