This is a general purpose linear triangular shell element. For the node numbering and the direction of the normal to the surface the reader is referred to the quadratic six-node shell element (S6) in Figure 70 (just drop the middle nodes).
In CalculiX, three-node shell elements are expanded into three-dimensional C3D6 wedge elements. The way this is done can be derived from the analogous treatment of the S6-element in Figure 71 (again, drop the middle nodes). For more information on shell elements the reader is referred to the eight-node shell element S8.