Thanks but I don't even want a remote terminal.
I just want the Pi3 to start mining when its turned on without any intervention.
I assume this can be done?
You must not have read that otherwise, you would realize that it does exactly what you are asking for. No keyboard. No monitor. Or with keyboard and monitor. Does not matter. Both methods to access the pi3 will still allow the instance of BFGMiner to run each time it boots.
Anytime it starts it automatically runs rc.local and will run that command line you placed in there in a detached screen. If you SSH into the pi3 you would simply type in screen -r and it will bring up the process. When you are done looking at it just click on the 'X' to close the screen.