Forcing DX9 is exactly what you have to do, G3D should work that way.
Just to be sure, please check first whether there is a d3d9.dll present in your Dirt 3 directory. Remove it if you find one.
If you checked that, and it still doesn’t work, try to create a vorpX shortcut for Dirt 3 (see the trouble shooting sticky for details). Despite not working correctly with some Steam/Uplay/Origin games these shortcuts hook more reliable than the auto detection.