From the quick start type guide on that I'm not sure it knows how to zone the ports as anything other than all green, so you may need something else to deal with the security zoning if you want to split the LAN into multiple zones.
Perhaps a PC with 3 network interfaces and a distro
such as smoothwall or IPcop. As others have stated you may be better just making sure the PCs are secure if there are not many of them.
Jambo