How to unlocked bootloader of MI A3 :-
1) Download Minimal ADB and Fastboot exe- download here
software on pc or laptop and install it on your pc or laptop.
2) Then hold shift key and press right key same time on your window pc or laptop. Then
select "Open command window here'' popup will appear to launch cmd.exe window on pc or laptop.
3) After that Go to MI A3, Setting=> About Phone=> Build number continuously press on "Build number" for 7 (seven times) until you see the message of "now you are developer".
4) And then Go to again MI A3 mobile Setting=> System=> Advance=>Developer Option=> USB debugging => enable USB debugging option, OEM Unlocking => enable OEM unlocking on your MI A3 Phone.
5) Now switch off your MI A3 mobile .
6) After that press & hold the power
button and volume down button same time on your MI A3 Phone.
7) Now your phone gone into fastboot mode.
8) Connect your phone to the pc or laptop using USB cable while it is on fastboot mode.
9) Launch the cmd.exe command window on pc or laptop ( as per column no.-2 )
10) After launching cmd command window type following text on cmd command window
and then press enter key, it show devices attached.
11) Again type on cmd command window on your pc or laptop.
fastboot reboot bootloader
12) Again type on cmd command window on your pc or laptop.
fastboot flashing unlock
and press enter key, the fastboot logo will automatically change and the process will erase your phone contents and then your phone will reboot.
13) Again after rebooting, press the power button and reboot your phone. While the device turned off, press and hold the volume down key and enter the fastboot mode again.
14) Again type following text to unlock critical partition on cmd command window,
fastboot flashing unlock_critical
15) Congratulation now your phone has been Unlocked completly and you can see Unlocked icon on your Mi A3 phone at the time of boot.