>Yes, that is the idea.
>
>But all three tables are in Sql Server.

Then you must create a trigger as Jake said, or SP wich will handle this for you., something like:

CREATE PROCEDURE InsertIntoTable3
@Description as varchar(8000) = ''
AS
BEGIN
     DECLARE @lcCode1 AS char(5)
     DECLARE @lcCode2 AS char(5)
     DECLARE @lnError AS int
     BEGIN TRANSACTION
        UPDATE TABLE1 SET @lcCode1 = Code1 = CAST(CAST(Code1 as int)+1 as char(5))
        @lnError = @@ERROR
        IF @lnError = 0
           BEGIN
               UPDATE TABLE2 SET @lcCode2 = Code2 = CAST(CAST(Code2 as int)+1 as char(5))
               @lnError = @@ERROR
               IF @lnError = 0
                  BEGIN
                      INSERT INTO Table3 VALUES (@lcCode1, @lcCode2, @Description)
                      @lnError = @@ERROR
                  END
           END
      IF @lnError = 0
         COMMIT
      ELSE
         ROLLBACK
END

(not tested)