instead of having 3 xbox... ones just changed the start to 3390 and the end to 5555 on the first one and it will open all the ports and the ones inbetween... thats 2 spaces... and do the rest to then others.
If I'm right, just have a firewall on your computer for extra protection if you're afraid of having ports open? Most routers like linksys have a firewall already installed in them.
If you want, you can set your Xbox in the DMZ, thus clearing up all of the port slots it's taking up. This will pretty much have no security issues, as most people don't go around making viruses for the 360. You could just set them in a range as one of the above users suggested, but a range that wide might cause conflicts with other programs trying to use those ports. Alternatively, you could put your computer in the DMZ so you don't ever have to worry about forwarding ports to that again. The downside will be that you lose the router's firewall, but if you have one on your PC, that shouldn't really matter. The DMZ basically removes the router's protection going towards a computer/device and thus makes it almost as if you had it plugged in directly to the modem. Hope I was of some help.
