bass1958,
Welcome to the forum. The Interlink can only be used to link two units together. The third port is for a GPS receiver. You can have a third unit, but it will not be able to sink with the interlink.
In order to get the Enhanced GPS refresh rate on all units, you will need two GPS receivers. The 1st unit will have its own GPS receiver and then connected to port 1 of the Interlink. The two bow units can be connected to the front GPS receiver with the AS-SYSLINK Cable, then one of those units will be connected to port 2 of the interlink.
Hope that helps.
Robert