Ive got a netgear but a different model, so i assume you set the ports up as Services 1st and then add them in to the Firewall to be opened to an ip address or range of address's.
If this is the case, maybe that 3074 port has already been defined as a service of some sort and you should be able to find it in the drop down menu of service's (ports) in the firewall when you add an entry. I had a similar thing with the 53 port as it was already defined as a service.