According to https://support.apple.com/en-ca/HT204893, users should be able to use an ATV3 or ATV4 as a HomeKit Hub but I can't get my ATV3 to be recognized by the Home app.
Additionally, it only lists the ATV4 and iPad as possible hubs. Did they change their requirements?
Control your accessories away from home
If you have an Apple TV (3rd generation or later), you can control your accessories with Siri commands when you're away from home. Just sign in to iCloud with the same Apple ID on your iOS device and Apple TV.
If your remote access isn't working, sign out of iCloud on your Apple TV, then sign back in. To sign in, go to Settings > Accounts > iCloud.
Additionally, it only lists the ATV4 and iPad as possible hubs. Did they change their requirements?