Files with w3u file extension can be found as one of the supporting files for Warcraft 3 custom maps. Custom unit information file.
Software that open w3u file
W3U file extension - Warcraft 3 custom units data
What is w3u file? How to open w3u files?
File type specification:
The w3u file extension is mainly related to Blizzard's Warcraft 3 real-time strategy game.
W3U format stores any custom created units inside a w3m or w3x map.
Units are created in the world editor, and then generates to this w3u file.
This w3u file type entry was marked as obsolete and no longer supported file format.
This type of file is no longer actively used and is most likely obsolete. This is typically the case for system files in old operating systems, file types from long discontinued software, or previous versions of certain file types (like documents, projects etc.) that were replaced in higher versions of their original programs.
The default software associated to open w3u file:
Company or developer:
Blizzard Entertainment, Inc.
Warcraft III: Reign of Chaos is a real time strategy video game released by Blizzard Entertainment in 2002. It is a third game from popular Warcraft franchise.
Warcraft III: The Frozen Throne 
Company or developer:
Blizzard Entertainment, Inc.
Warcraft III: The Frozen Throne is a real-time strategy computer game developed for Microsoft Windows, Mac OS and Mac OS X by Blizzard Entertainment and released in 2003. It is the official expansion pack to Warcraft III: Reign of Chaos.
List of recommended software applications associated to the .w3u file extension
Recommended software programs are sorted by OS platform (Windows, macOS, Linux, iOS, Android etc.)
and possible program actions that can be done with the file: like open w3u file, edit w3u file, convert w3u file, view w3u file, play w3u file etc. (if exist software for corresponding action in File-Extensions.org's database).
Unspecified and all other actions for computer programs working with w3u file - Warcraft 3 custom units data
Click on the software link for more information about Warcraft III: Reign of Chaos. Warcraft III: Reign of Chaos uses w3u file type for its internal purposes and/or also by different way than common edit or open file actions (eg. to install/execute/support an application itself, to store application or user data, configure program etc.).
Microsoft Windows:
Main software associated with w3u file by default:
Warcraft III: Reign of Chaos
Warcraft III: The Frozen Throne
