What is 3DNUS? 3DNUS allows you to download NUS content for the Nintendo 3DS, and pack it as .cia(s) if you wish. The application is simple and easy to use. It's light, and works quickly.

This application is only for Windows based Computers. This is not designed for use on the 3DS.

Quoted from GBAtemp:

For anyone who is unaware: 

Nintendo has recently been able to patch the way 3DS' grab titles from the CDN and otherwise. 
This takes effect for firmwares 11.8+. 

Soon, Nintendo may pull the plug for downloading content from the CDN's that are NOT a 3DS. Meaning, 3DNUS will most likely become useless and obsolete. 

If you need to download any firmwares or titles; please grab the latest release and download what you need immediately!

• Download NUS content and pack it as a .cia
• Download Titles (System Titles only)
• Download Full Firmwares
• Spoof Titles or Firmwares
• Automatic Updates for the Application; to ensure you have the latest experience.
• Ability to Dump the LOG of the Application; which can help me, and others help you if a issue has occurred.
• Download NUS content for both new3DS and old3DS.

To download a firmware, type the Version as such "XX.X" then followed by either "U, E, or J." U for USA, E for Europe, and J for Japan. Then Check Silent (If you want to ignore Download and packing errors; also allows the app to finish downloading the remainder of the Firmware.

To download a title, do as follows: 000000000 (Title ID) followed by the version. For example: 000000000 v0000. Then download, and pack it if you wish.