Reminder: HomePod can replace an Apple TV or iPad as a HomeKit hub
Not only can Apple's HomePod be used to control HomeKit devices, but it can take the role of a HomeKit hub, allowing for access and control of compatible peripherals from outside the home.

The HomePod is third class of device that can act as a HomeKit hub. The HomeKit hub feature was previously limited to the Apple TV fourth generation and newer for all HomeKit functions, or an iPad capable of running iOS 11.
Just using an iPhone will also allow you to control these devices, rooms, or scenes with Siri.
Activating your devices manually outside of the home, sharing access to your HomeKit setup to other users, or setting up a series of HomeKit responses to do something at a certain time of day or in response to a stimulus from a sensor requires a home hub.
The third generation Apple TV can be used as a HomeKit hub, but it won't be able to be used to remotely access HomeKit cameras, change user permissions, or create automations from the device.

The HomePod is third class of device that can act as a HomeKit hub. The HomeKit hub feature was previously limited to the Apple TV fourth generation and newer for all HomeKit functions, or an iPad capable of running iOS 11.
What you can and can't do in HomeKit without a home hub
Without using an Apple TV, an iPad, or a HomePod as a home hub, you can organize devices together in a room, control accessories, and create a scene to control multiple accessories with a single tap when you're in range of your wireless network.Just using an iPhone will also allow you to control these devices, rooms, or scenes with Siri.
Activating your devices manually outside of the home, sharing access to your HomeKit setup to other users, or setting up a series of HomeKit responses to do something at a certain time of day or in response to a stimulus from a sensor requires a home hub.
The third generation Apple TV can be used as a HomeKit hub, but it won't be able to be used to remotely access HomeKit cameras, change user permissions, or create automations from the device.
Comments
But it's this little sort of thing - why the he11 is Apple offloading the Home app when it's designated a "hub" and is needed??? With usual lack of help from Siri, one is left to ones own devices to debug these things. With an iPad, at least there's a user interface, but what do you do if you designate HomePod as your hub and something goes wrong???
As an aside: are you sure the third generation Apple TV can be a home hub? This is the last version before the "Siri" enabled Apple TV, right? I have two of those - and I looked for a "hub" menu item when I started my automation journey and didn't see it. I believe I read somewhere that Apple disabled it for 3rd-gen on some upgrade to tvOS? I might be wrong - will check again once I get home.
Second aside: there's no clear tutorial on what you need to do to get geo-fenced based automation working for multiple devices. Currently, only my iPhone turns on the lights when I get home. If my wife comes home, nothing happens. Related: there's a "When Anyone Leaves Home" - why isn't there a "When Everyone has left Home" - a much more useful condition? Where is all the home automation documentation???
(Vaguely related): Anything to do with the HomePod product launch?
Yes, why? Looked for news I may have missed, but nothing "big"...
So maybe it's because of the HomePod, but strange. Think the news about health data would be a (great) positive!?
I have not yet been able to set up an ATV3 as a hub. Does not show up in the Home App and nothing in the ATV3 iCloud setting mentions Home as the web site says it should. Everything is updated to latest rev. 2-factor authentication is active.
Anybody help solving this would be greatly appreciated. Thanks.
As a growing Homekit house, it's very reliable for me!!! I'm almost always controlling everything with my Apple Watch. No problem controlling away from my house either. Using the iPad never made much sense to me as a Hub. For me, my semi new 12.9" iPad Pro comes with me to work and other places. If I take it with me, it's not going to work at all as a Hub for the home.
My Apple TV 4 is my Homekit Hub. If I remember right, there's a setting to turn it on.
As for Homekit on a AppleTV 3, look at this link and maybe it'll be of some help
https://discussions.apple.com/thread/7904164
Did you go into the HOME app and add you WIFE to your Homekit settings? If you go into the HOME app, on the top left is a arrow, click on that, then scroll down to People and click on Invite and add your Wife and anyone else here.
...on second thought, maybe I don't want to know!