@fred
I asked a related question a couple of weeks ago. 3D Lines appears to have a particular behaviour related to the z depth - they do not conform to any perspective. The 3D Lines that are generated remain the same stroke width no matter the z distance from the virtual camera. I have also found that using layers and line size was the only way to simulate perspective and distance occlusion. But that is not going to work for every situation.
Best Wishes
Russell