"About the macOS High Sierra 10.13.3 Update
The macOS High Sierra 10.13.3 Update improves the stability and security of your Mac and is recommended for all users.
This 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

Which update should I install? (See download links below)
Choose the macOS High Sierra 10.13.3 update that corresponds to your Mac:

If you have an iMac Pro, install the macOS High Sierra 10.13.3 iMac Pro Update.

If you have another Mac running macOS 10.13.2, install the 10.13.3 Update.

If you have another Mac running macOS 10.13 or 10.13.1, install the 10.13.3 Combo Update."

Download pages for macOS 10.13.3 & Security Update 2018-001 updates:

• Download macOS High Sierra 10.13.3 Update (2 GB)
  
  
• Download macOS High Sierra 10.13.3 Update for iMac Pro (1.53 GB)
  
  
• Download macOS High Sierra 10.13.3 Combo Update (2.25 GB)
  
  
• Download Security Update 2018-001 (El Capitan) (863 MB)
  
  
• Download Security Update 2018-001 (Sierra) (766.6 MB)

Related Apple Articles:

• About the macOS High Sierra 10.13.3 Update
  
  
• About the security content of macOS High Sierra 10.13.3, Security Update 2018-001 Sierra, and Security Update 2018-001 El Capitan (includes "Meltdown" patch for macOS 10.12.6 and OS X 10.11.6 - High Sierra already patched in previous update.)
  
  
• About the security content of Safari 11.0.3
  Available for OS X 10.11.6, macOS 10.12.6, macOS High Sierra 10.13.3

FYI on Nvidia Web Driver Updates:
Users of Nvidia's web drivers should know that new OS updates (versions or later build numbers after Security Updates) break any currently installed Nvidia web graphics driver. I'd switch to the default Apple driver and reboot before updating, if your card is supported with the default drivers. If not, wait for a Nvidia driver update. (Some non-Mac EFI cards are not supported by default OS X drivers.)
Of course have a current backup (and if possible, a Clone) of your boot drive before any update.
(As usual, I'll post download pages for Nvidia updates when spotted.)
FYI: See Mac Nvidia drivers page for macOS 10.13.3, 10.12.6 and OS X 10.11.6 builds with Security Update 2018-001. Also a new CUDA 387.128 update. (For later updates, check the macOS updates page, Mac Nvidia Driver Downloads page, and recent site updates.)

iOS 11.2.5, tvOS 11.2.5 and watchOS 4.2.2 Updates

• iOS 11.2.5 release notes
  - Includes HomePod support, Siri News and Other improvements & fixes. (See linked page.)
• About the security content of iOS 11.2.5
  
  
• tvOS 11.2.5 release notes
  - "includes general performance and stability improvements."
• About the security content of tvOS 11.2.5
  
  
• watchOS 4.2.2 release notes
  - contains new features, improvements, and bug fixes. (See linked page for details.)
• About the security content of watchOS 4.2.2