This is not a good choice for a serial number to be used as the table's primary key in relationships to other related tables. A simple auto-entered serial number is better for that purpose.
Use two fields, one for the year and one for the serial number sequence 1-999. Set up a self join that matches records by the year fields. Use the Max function to return the largest serial number value of the record with the same year and add 1 to assign a serial number to a new record.
Note that most methods used for assigning such a value risk generating duplicate values if more than one user is creating new records at the same time.