Solution or Workaround
When I import a 3D AutoCAD DWG file into ArcView, the values in Elevation field in the attribute table of the CAD theme are set to zero. Why don't the elevations come into ArcView?
The entities in the drawing are 3D polylines. Because 3D polylines can have different z values at each vertex (x,y point), there is no way for ArcView to assign a single elevation to each entity. Therefore, the CAD Reader cannot import the elevations from this type of entity. Elevation is an older entity property for 2d objects and doesn't really have anything to do with the geometry of that entity or the newer entities that are 3D.
If the 3D polylines you have created have identical z values and there is no other reason to use 3D polylines, you can use 2D lwpolylines (lightwieght polylines) and assign a z-value to each. A 2D lwpolyline with a z-value has identical z values at each vertex. The CAD Reader takes the AutoCAD group code 38, which is the elevation, and assigns it to the Elevation field in the CAD theme's attribute table.