The 3D preview is not designed to show the surface maps you have created. It uses a generic, slope-based terrain coloring to help show you the slope and shape of the terrain. The way that the TG 0.9 and the 3D preview works it would actually be extremely computationally intensive to try to represent the surface maps in that view. It's just a limitation of TG 0.9 and it's not something that will change as development on TG 0.9 has ceased. TG2 has a more accurate, though somewhat slower, 3D preview, which does include surface mapping and much more.
- Oshyan