Multiple subnets with same WAN on C3510XZ

TZ0a

First I need to clarify that my question has nothing to do with the NBG/ARMOR series but that’s the option that this forum selects for me when I select “home router”. 

I’d like to setup multiple subnets on my C3510XZ which I’m using with CenturyLink fiber. I’ve figured out how to change the LAN settings to assign an IP range and DHCP per interface grouping. The problem I’m having is that when I create a new interface grouping, the WAN selections are “consumed” by the others. I only have one WAN connection so that’s consumed by the first grouping:

The others then don’t even show a drop-down with WAN options to select and can’t be saved as valid configs:

Is it possible to create multiple interface groupings that all use the same WAN / (all connect to the internet)?

If not, is there a workaround that would allow me to setup multiple subnets all connected to the internet? My objective is to create three subnets based on LAN port and/or WLAN/SSID. Thank you.