The way I would try to achieve this is to first install all 8 ports on the first system through the RealPort wizard (when you install the RealPort driver). Then afterwards, drill down the the RealPort properties and unselect the last 4 ports, leaving only the first 4 selected/checked. By drill down in the properties, I mean double-click that device in the MultiPortSerial Adapters category to get into the RealPort properties. I forget where exactly, but if you dig a bit, you should find an area where you can select what ports to actually use.
Then repeat this process on the other 2 systems, except only check the single ports you want each of them to use. For example, if you selected ports 1-4 on the first system, then select only port 5 on the 2003 B system, and only port 6 on the 2003 C system.
Does this make sense?