I had previously installed EmuELEC on the TV box, and later trying to install Armbian proved difficult because the device kept booting into Android recovery mode. I learned that installing EmuELEC even once writes data to the firmware that complicates installing Armbian on my TX3 Mini; the solution was to perform a factory reset via Android and then proceed with the installation.
I hope this helps anyone else who is struggling to get the system to boot for the first time.