Canon EOS-1D X Mark III Advanced User's Manual page 521

Steps on the camera (2)
11.
Select [OK].
12.
Press the <
Caution
Bluetooth connection consumes battery power even after the camera's auto power
off is activated. Therefore, the battery level may be low when you use the camera.
The camera can be paired with one smartphone. Before pairing with a different
smartphone, clear the information about the currently paired smartphone (
Keeping pairing records for previously paired cameras on your smartphone will
prevent it from pairing with this camera. Before you try pairing again, remove
pairing records for previously paired cameras from your smartphone's Bluetooth
settings screen.
> button.
Pairing is now complete, and the camera is connected to the
smartphone via Bluetooth.
A Bluetooth icon appears on the main Camera Connect screen.
521
).