Initially, I connected the Google WiFi as outlined in the instructions via the app and thought this is a piece of p*** to set-up...... As I said, everything seemed to be working OK until I attempted to put the Hub 3.0 into Modem only mode. From my understanding, with the Hub 3.0 it does not matter which port you plug the external router into, but for your information, I had it plugged into the top port (furthest away from the co-axial cable).
When I put the Hub into Modem only mode, everything seemed OK, in so far as the Hub rebooted and eventually the only light that was on was the solid magenta light. At this point the Google WiFi was showing no internet connection. I tried plugging the Google WiFi into other ports on the Hub 3.0 (rebooting each time) but still no internet. When I gave up and tried to revert back to when it was working, I was unable to connect to the Hub 3.0 setup page to take it out of Modem only mode. Even attempting a pin rest failed. Had to call the "help desk" who did something at their end to reboot the Hub 3.0 back into router mode.
I will try again when I get time over the next day or so, and document the steps I took and selections made - so hopefully if it still does not work, you (or somebody else) maybe able to offer a solution.