First thing you need to know if you want to officially unlock bootloader on your Xiaomi phone is that you must have Developer ROM installed.
Short information about different ROM versions:
Global MIUI Rom starts from 8, 8.3 or 8.3.1
Developer ROM version start from the year it has been released.
In case your Mi account is old enough, you may try to unlock even on a Stable ROM version, but this happens very rarely. In case you aren’t able to get unlock permission, then you are required to flash Weekly Developer ROM.
There are exceptions for a couple of devices: Mi4c and Mi Note Pro device comes with unlocked bootloader. You’ll need to flash China Developer ROM version, got bootloader locked again, and then request permission to unlock bootloader. Waiting time in this case is around 4-8 days, and all that time Mi Unlock will be showing “Current account is different from the account info on the device” message.
How to unlock bootloader after getting official unlock permission from MIUI
Sign in into your Mi account with enabled unlock permission. It’s recommended to make some random actions related to this account, for example sign in and find your device here http://www.i.mi.com/, this helps to avoid stuck at 50% bootloader unlock issue.
Download and extract MiFlashUnlock (download link below).
Start MiFlashUnlock and press “Accept”.
Then sign in with your Mi account with unlock permission.
Turn off your device completely.
Press “Power” and “Volume -” buttons and wait for rabbit logo to appear.
Now connect your device to PC and press “Unlock” to start the unlocking process.
If all three boxes are highlighted, then you’ve successfully unlocked a bootloader on your Xiaomi device! If not you should visit MIUI forum and ask for additional help.
On Redmi 3 & Redmi Note 3 device you can check bootloader status by going to “Settings” > “About” > tap 5 times on kernel version and look for fastboot lock state. If you see “unlock” then you have unlocked device, if “lock” than bootloader is locked.