I just recently was able to use my FC3 machine's remote desktop on another PC in my home network.
The "0" you're referring to isn't actually a port number, but the number of the VNC server (or, in this case, the remote desktop) in the host you're connecting to. If you were accessing your FC3 machine remotely using a VNC viewer, you would enter "host:0" in the VNC viewer. If, for example, you had more VNC servers running on your FC3 machine, they would be accessible via "host:1", "host:2", etc.
What you'll need to do is open 59xx:TCP in your FC3 machine, where xx is the number of the VNC server. I'm also using FC3's built-in remote desktop (and also not running another VNC server). That would make my remote desktop number 0. So in my system, I opened up 5900:TCP, and everything worked.
Hope that helps.