set serveroutput on

-- get the first 3 characters
DECLARE
   v_3char   VARCHAR2 (3);
BEGIN
   FOR i IN  (SELECT long_col FROM mytab)
   LOOP
      v_3char := SUBSTR (i.long_col, 1, 3);
      DBMS_OUTPUT.put_line (v_3char);
   END LOOP;
END;
/