DXTBmp will convert the CFS3 textures for editing. I no longer have CFS3 installed but I imagine the actual repaintng is basically the same as for the other M$ sims.

I can't give specific instructions for CFS3. There might be a tute I don't know about & you can browse through the existing threads on this forum to get the general idea. The principle will be much the same as for any other M$ sim. First, I suggest you download the latest DXTBmp & check it out. http://www.mnwright.btinternet.co.uk/programs/dxtbmp.htm
DXTBmp Help is very useful & gives information on the various FS texture formats. Basically you open a texture in DXTBmp & send it to a graphics editing program for editing. Then save your work before converting it back to the CFS3 texture format. Always take backup copies of files before editing them.