This deployment is not correct. You need to create all layer 3 vlans, filter traffic between the three vlans, and route all internet bound traffic through a point to point between the firewall and a layer three interface on the switch. You could also create three layer 2 vlans, doing router on a stick with the firewall, assuming the firewall understands vlan tagging or you need to put a router between the firewall and switch and do router on a stick. Either way you need to deny all traffic inter-vlan.
I don't think Smoothwall can handle vlans so I'll need to make another plan or get another type of firewall. Can you suggest an open source firewall or other?
Anyway, I've now played with two setups, one as detailed by the thread from spiralclimbing and one where I setup the Netgear L3 to handle multiple vlans with inter-vlan routing.
The inter-vlan routing option is nice because it allows us to have various subnets but I'm not sure its the otion as the other subnets have access to all services on the main + internet subnet, I think due to the inter-vlan routing function.
The otion from spiralclimbing is more strict and keeps the vlans locked down due to no inter-vlan routing but all vlans are on the same subnet.