You cannot apply three-dimensional formatting to some kinds of shapes, such as beveled shapes or multiple-disjoint paths. Most of the properties and methods of the ThreeDFormat object for such a shape will fail.
Use the ThreeD property to return a ThreeDFormat object. The following example adds an oval to myDocument and then specifies that the oval be extruded to a depth of 50 points and that the extrusion be purple.
Set myDocument = ActivePresentation.Slides(1)
Set myShape = myDocument.Shapes _
.AddShape(msoShapeOval, 90, 90, 90, 40)
.Visible = True
.Depth = 50
'RGB value for purple
.ExtrusionColor.RGB = RGB(255, 100, 255)