There is no firmware download for the Avalon Nano 3. The website you are referring to is providing only the recovery tool!
Have you tried to download the recovery tool you should be able to find the firmware once you extracted the files from the recovery package included in the recovery tool for nano 3?
Also, it includes a video guide on how to install the driver and flash the unit with the firmware image.
Here's the extracted files from the recovery tool
I know, but did you check which version you would install with that? It was an old version.