You may have a few options. If your G3 skin is not the Base Female or Male UV, you could use Map Transfer to convert it and get one of the products by Cayman or Sickleyield to map the skin to G9. If it is the base UV then you could directly use those products without conversion.

I just happened to find a product that I forgot I bought called "ultimate unwrap 3d pro" which does conversions from G2 to G8. You would need to save an obj file to disk with all of the textures for the source and an obj for the target and the program projects the textures. The only issue with that product is that both the objects need to be the same shape and make sure as much of the mesh is exposed (open mouth, spread fingers and toes). Also you would need to move eyes from the base mesh on G3/G8 so it doesn't smear the textures. So it's a bit of work to get started, but does convert the textures. The product does have a tutorial on their website that explains most of the process. ("Tutorial 3 for Repainting Mesh UVs (Texture Transfer)")

The first option would probably be the easiest.