Thank you for your prompt reply.
In all cases, I was able to pair my band with Gadgetbridge. I could also see that the band was shown as "connected" in Gadgetbridge, and that the app was able to communicate with my Mi Band 3.
However, everytime I flashed the custom firmware, I would get the "Connect to app to update" screen on my Mi Band 3, even though the band was shown as connected and syncing correctly with Gadgetbridge...
So I am now back to the original firmware, looking for a solution to update the band and get a custom firware working