In case of COM3 Port is busy error while flashing TWRP on locked bootloader device. Here’s a solution that might work for you.

Solution for “0х800700aa: open serial port COM3” – Flash TWRP on Xiaomi Redmi 3

  1. Run MiFlashClean.cmd (should be in folder with MiFlash.exe)
  2. If you can’t find MiFlashClean.cmd, then here’s the code inside of this file:
    del /q MiFlash-*.log
    rundll32 qcCoInstaller.dll,qcInitializeSetting
  3. Right click on MiFlashClean.cmd and select “Run as administrator”.
  4. Try to flash TWRP again, error should be fixed.