Get the .TIN File
Currently, it is the responsibilty of the Survey Branch to provide the .TIN file to design. Go see Ray Shumway in Survey. If you're the inquisitive type, he may show you how this is done. We may need to have Ray write a brief explanation of the procedure and replace my rambling here with it.
You must have this file to make the 3D triangle file!
I do go into some detail about creating a TIN file from your Proposed Design in the section covering Passing Sight Distance, but... what you need first, is a TIN file that represents the existing condition of the ground.
More coming later...