Can the Balance 310 support more than one USB Modem (i.e. using a hub).
As far as I know, it is not possible to do it.
However, you can use a 3G/4G USB router with LAN output that you can connect to one of the 3 available WANs in 310.
nuno is correct, it only supports one USB modem. If you need to support multiple USB modems you could look at the MAX 700 or MAX On-The-Go routers.