/*
 *  call-seq:
 *     dvector.each3_with_index(other1, other2) {|x,y,z,i| block } 
 *  
 *  Calls <i>block</i> once for each element in _dvector_, passing that
 *  element as a parameter along with the corresponding element from the _other1_ and _other2_ vectors and the index.
 *  The three vectors must be the same length.
 *     
 *     a = Dvector[ 1, 0, -1 ]
 *     b = Dvector[ 3, 4, 5 ]
 *     c = Dvector[ 6, 9, 2 ]
 *     a.each3(b, c) {|x,y,z,i| print "(", x ",", y, ", ", z, ",", i, ") " }
 *     
 *  produces:
 *     
 *     (1,3,6,0) (0,4,9,1) (-1,5,2,2) 
 */ VALUE dvector_each3_with_index(VALUE ary, VALUE ary2, VALUE ary3) {