If a game doesn´t support widescreen it usually will not work.
But in case of Warcraft 3 i found this:
Here is a guide to setup War3 in Widescreen for Windows:
1) Make sure the resolution you want to run is supported by your graphics drivers.
If you can select a resolution in your Operating System, then it is supported. If the resolution doesn't exist, read our guide on how to configure custom resolutions in windows.
2) Backup your registry, in case something goes wrong.
START > Run > regedit > file > export