I at first only had a micro-USB OTG cable. I also had a microUSB to USB-C adapter.
That combination does NOT work. You cannot use a micro-USB OTG cable with a micro-USB to USB-C adapter, at least not with FP3.
A genuine USB-C-OTG (aka USB-A female to USB-C) does work. I was able to operate FP3 with both a mouse and USB keyboard so far.