; (2) the labor organization and employer engaged in their first bargaining session following the issuance of a bargaining order by the Board; or (3) the labor organization and successor employer engaged in their first bargaining session following a succession. (g) The Board shall dismiss any petition for an election with respect to a bargaining unit or any subdivision if there is in effect a lawful written collective bargaining agreement between the employer and an exclusive representative covering any employees in the unit specified in the petition, unless the petition is filed— (1) on or after the date that is 3 years after the date on which the collective bargaining agreement took effect