I just did some research on this, so I will answer my own question:
The Aerotank adjustable airflow control valve can be used on both the Protank 2 and the Protank 3.
If you use the Aerotank adjustable airflow control valve on a Protank 2, use only the standard replacement coil cartridge designed for the Protank 2.
If you use the Aerotank adjustable airflow control valve on a Protank 3, use only the standard replacement coil cartridge designed for the Protank 3 and Aerotank (both use the same replacement dual coil cartridge).