Apple Support lists the following fixes in the 10.13.3 update:
- Addresses an issue that could cause Messages conversations to temporarily be listed out of order
- Resolves an issue that could cause your Mac to stop responding when connected to an SMB server
It also patches the bug that allowed the App Store menu in System Preferences to be unlocked with any password.
It’s in the Mac App Store now.
If you’re running macOS Sierra or El Capitan, Apple has pushed “Security Update 2018-001” that addresses the Meltdown vulnerability. If you’re on 10.11 or 10.12, run updates today.