I recently bought an iPhone 6s that was locked to three, I bought a Virgin sim and the phone is still locked. The problem is that the carrier is not supported under the activation policy that is currently assigned by the activation server.
Welcome to the Community Forum and thanks for your post. I'm sorry to hear that you are experiencing a problem with unlocking your iPhone 6s that's been locked to the Three network. Has this been sorted out for you since your post?
If you are still having problems with this, please let me know and I'll look into this for you.