(defun fill-pl (pts / n pc)
  (setq n (float (length pts))
       pc (list (/ (apply '+ (mapcar 'car pts)) n)
                (/ (apply '+ (mapcar 'cadr pts)) n)
                (/ (apply '+ (mapcar 'caddr pts)) n)))
  (while (>= (length pts) 2)
         (entmake (list (cons 0 "3DFACE")
                        (cons 10 pc)
                        (cons 11 (car pts))
                        (cons 12 (cadr pts))
                        (cons 13 pc)))
         (setq pts (cdr pts)))
  (princ))