I've successfully unlock bootloader of RN3 officially after trying for 3 hours. There are 2 guides that help me the most.
In my scenario, based on my observation, the reasons that lead to 50% stuck is NOT due to xiaomi error per se, but it's due to PC not able to recognize "Android ADB Interface". Once the driver is installed, and the debugging bridge is able to link, unlock process just proceed smoothly.

I received the permission to unlock sms within 30 minutes!!! so fast.

The 2 most important guides (I don't follow exactly)

Guide 1) http://xiaomitips.com/guide/how-to-fix-cou...unlock-process/
- I skipped Step 3 & 8 & 9
- I disable "Verify apps over USB" in developer option
- still stuck at 50% after trying many times

Guide 2) http://en.miui.com/thread-136188-1-1.html
- after successfully install the adb driver, within 10 seconds the phone is unlocked

Just give Guide 2 a try, it may save many of your time.

additional observation:
I noticed that when I connect RN3 to PC, the default is "USB for charging" and I don't see Redmi Note 3 in Windows Explorer.
I have to choose "Transfer Files (MTP)" to access RN3 storage (Windows Explorer show Redmi Note 3).

After I install the above adb driver, the Redmi note 3 is shown in Windows Explorer before before i click MTP.

I guess this maybe the root of the problem. When in fastboot mode, there is no option to enable the linkage between PC & phone. After adb driver is installed, the linkage is automatic as soon as USB cable is plugged in.

All marshmallow roms should me mostly stable now, they just differ in terms of customization options and features. I would still highly recommend you to try out Nougat some day if you want to cause it gives quite some performance increase since Vulkan is enabled now and GPU blobs from Mi Max Nougat Beta are used.