C3s are mesh/animation data
dds are textures
You can edit/view DDS images very easily using photoshop or a number of other photo editing programs.
C3 files are a bit more tricky... I know ultimation released his viewer a while back and I believe it works fine for most things but it's not an editor and I don't believe it will work with things like glows/effects.
In most cases the c3 files for effects is just a plane on which a texture is mapped.
EG: