I do not use Alexa/Siri for sensors.
I do use Dashboards for all my sensors.
Link to Hub is sending all my devices but not all of their attributes to the 3rd Hub, as discussed elsewhere.
I have all my real devices on at least one Dashboard. All my motion sensors are on two Dashboards, for example. I have a Dashboard just for Motion Sensors and the Dashboard for Downstairs / Upstairs have the same motion sensors.
I focused on Dashboard to get my imagination around what was available in this triple hub architecture. My thinking was: If I can't see it in Dashboard, I won't be able to see it in Homebridge or Amazon Echo apps.
However, I only use Homebridge and Alexa for my switches and outlets. In other words, if I want an overview of my system, I look at dashboard. If I want to do something, open close a lock, turn off a light, etc, I use iOS Home app.
In other words, I can see that the status of my contacts, motion sensors, temp, tilt, on and on, are reflected on Dashboard, which is, for me, only running on the 3rd hub 'coordinator.'