Hi JCROMP,
Thanks for coming back to us about this issue with your Brother's SIM card, I am sorry it's turned out to be a difficult task!
As you've said the device was purchased from Three, the handset may still be locked to that network. Do you know if you have spoken to Three about unlocking the handset previously?
We would be unable to unlock it from our side as we can only unlock handsets that are purchased from ourselves. So if you're not sure, I would get in touch with Three and get them to investigate further 🙂
Thanks,
Megan_L