Unfortunately it is not possible to charge the battery grip via USB-C. You can see the top right green LED is on when I charge the camera itself via USB-C, indicating that the internal battery is being charged:
The two green LEDs bottom left that indicate the batteries in the grip are being charged only switches on when the dedicated charge cable for the grip is plugged in (the black cable bottom left):
3 thoughts on “Charging the Fujifilm XT-3 vertical battery grip”