Have you updated the drivers to the newest version? Also ATI cards tend to have no issues when you run WC3 in OpenGL mode (instructions at
www.blizzard.com)
http://www.blizzard.com/support/?id=mwr0512p
Step 4
Try OpenGL or Software Transformation and Lighting (of polygons):
1. Click Start
2. Click Programs
3. Click Warcraft III
4. Right-click on the Warcraft III icon
5. Select Properties
6. Add -opengl to the Target field. (Example "C:\Program Files\Warcraft III\Warcraft III.exe" -opengl)
7. Click OK and try the game again
8. If you still experence problems repeat the above steps but replace the -opengl tag with -swtnl and try the game again