       intersect3 lista listb
              Procedure  to  intersects two lists, returning a list containing
              three lists:  The first list returned  is  everything  in  lista
              that wasn't in listb.  The second list contains the intersection
              of the two lists, and the third list contains all  the  elements
              that  were  in  listb  but weren't in lista.  The returned lists
              will be sorted.
