There is only one file type that uses the .W3N extension:
Warcraft 3 Expansion Campaign Map File
A W3N file is used by Warcraft III: The Frozen Throne, an expansion to Warcraft III. It contains campaign maps, which consist of a series of missions and is used for creating custom campaigns. W3N files may be created using the Campaign Editor component of the Warcraft III World Editor but may only be used by The Frozen Throne expansion pack.
To install W3N campaign maps place them in the "Campaigns" folder not the "maps" folder. Then select "Custom Campaign" from the single player main menu to play the missions. Some examples of campaign maps include Day of the Dragon, Lord of the Clans, and The Last Guardian.
Updated: March 1, 2017