Install this application on your smartphone you can find your smartphone with a double tap on the Mi Band 3.
Also if you can't find Mi Band this app helps you to find it.
Move around and the signal level will tell you the right direction and the vibration of the band will tell where it is.

For the first run you need to turn on the discoverable mode on the bracelet through the end of the application.

The application requires Mi Fit or Mi Band Master or Notify & Fitness (so that the bracelet is already connected to the smartphone).
After connection the discoverable mode on the bracelet can be turned off. The application will remember the Bluetooth address of the bracelet and will connect to it directly.