If your device should connect to A.net, B.net or C.net, the device needs to be registered in each IoT Hub instance. Azure IoT Hub Device Provisioning Service can do this for you if you link each IoT Hub instance to your DPS and create a proper allocation policy defining the target IoT Hub instance for your specific device based on your requirements (e.g., load balanced, closest distance, canary instance, et cetera). This is the recommended way as it simplifies management and device provisioning.
If you want to do this manually, e.g., in an active-active or active-passive HA scenario, you need to create an identity registry entry for this specific device in each IoT Hub. If you have many devices what should be able to connect to multiple IoT Hub instances, you might consider exporting the device registry of IoT Hub A.net after creating all devices and importing the device registry into IoT Hub B.net and C.net. If you add or remove a device, you need to do this in each IoT Hub instance, what could become cumbersome and difficult to keep track of.
For more information see also the IoT Hub documentation on identity registry
If your question is answered, please use "accept answer" and feel free to up vote the answer. This helps others to find the right answer more easily.