Hi, I've got my Hub 3 in modem mode and it's working fine with Google WiFi. However, only one of the ethernet ports appears to work at a time - ie to get my TiVo box to work I have to connect it to the Google node, not the Hub 3.0. Is that a fault, or how it's meant to work? Ideally I'd like to have my TiVo plugged into the Hub rather than the Google node if at all possible.
Modem mode disables the router and switching functionalities of the Hub 3, it's solely a modem in that mode and will only provide the WAN connection to the third party router you connect to it.
If you need more ethernet ports than the Google router provides you'll need to get a gigabit switch, they're reasonable cheap on Amazon, around £10 for an unmanaged 5 port switch. Just make sure it states 10/100/1000mbps or gigabit ethernet.
Google WiFi is a replacement for your hub, so you should have the hub in modem mode. As Google WiFi has only two Ethernet ports you can plug in an unmanaged gigabit network switch to add ports, see my sig for an example of one.