GT-View can only directly monitor a single inverter with full operational detail.
The GT inverters uses pins 2,3,5 in the DB9 connector , this needs to make it back to the laptop so it retains the 2,3,5 pinout ( straight thru connection)
On the laptop, you most lieky need a usb to serial converter
see this thread
http://www.solar-guppy.com/forum/viewtopic.php?t=392
If you interconnect the GT's using standard network cables, GT-Viwe can display the operational data of the inverter the laptop to GT is connected to AND also display system total ACWATTS and LIFETIME kwh values
If you desire more than this, you need to have three copies of GT-View running each connected to one of the 3 inverters in your case.
Many people find it easier to make ther own cables, as only three wires are needed for each laptop to inverter and there are 8 wires in Cat5 cables. Also, the pin 5 (Ground) can be shared, allowing a single cat5 to be custom wired to connect the 3 inverters to the laptop
Since your using a db9 to RJ45 connector, I dont know what actual wires are being used in the cat5 cable. I'm guessing, if you use the same brand db9 to RJ45 on each end , you should be OK