I'm not sure if this is possible - hopefully someone else will come in and prove me wrong...!!!
It is easy anough to produce a simple XYZ of the triangle points although the surface may not be exactly the same as the triangles are not exported.
Display the surface with points turned on and then Extract Objects > Points from the surface, then perform a data extraction or _MAPEXPORT on the Points direct to shp file (I've attached the dbf for one I just did)
neilyj (No connection with Autodesk other than using the products in the real world)
IDSP Premium 2014 (mainly Civil 3D 2014 UKIE SP1 & Infraworks with some limited 3ds Max Design) Win 7 Pro x64, 256Gb SSD, 300Gb 15,000 rpm HDD 16Gb Ram Intel Xeon CPU E5-1607 0 @ 3.00GHz (4 CPUs), ~3.0GHz NVIDIA Quadro 4000, Dual 27" Monitor, Dell T3600
If you already added your TIN to a surface definition, then create a Dem file with _SURFACEEXPORTTODEM, finally you convert the resulting .TIF (or .DEM) to .ASC format, by means of the opensource Microdem.