The game has native support for widescreen resolutions, just use the video options menu.
To get resolutions such as 1280x768, which isn't supported natively, or any other custom resolution use this method:
First locate your C&C 3 directory(Default: C:\Program Files\Electronic Arts\Command & Conquer 3 Tiberium Wars Demo) then right click on the CNC3Demo.exe, create a shortcut anywhere you would like.
Now right click on the shortcut and select properties. The Target box should read:
"C:\Program Files\Electronic Arts\Command & Conquer 3 Tiberium Wars Demo\CNC3Demo.exe"
Add a space and enter: -xres 1280 -yres 768
Or any custom resolution you would like to use.
Your Target box should now look like this:
"C:\Program Files\Electronic Arts\Command & Conquer 3 Tiberium Wars Demo\CNC3Demo.exe" -xres 1280 -yres 768
Most resolutions are supported natively, but some (such as 1280x768) are not. If your preferred resolution is not supported natively, then you'll need to edit options .ini.
For C&C3, it's located here:
C:\Documents and Settings\Administrator\Application Data\Command & Conquer 3 Tiberium Wars\Profiles\<yourName>
Find this line:
Resolution = xxxx xxx
And edit it to set your resolution.