As far as I am aware the A3 doesn't have a remote boot release (like that found on the A4/A5).
The way to open the boot is simply, use the fob and press the middle button until the boot releases or open the boot when actually er opening the boot (ie the button above the numberplate).
The way to open the boot is simply, use the fob and press the middle button until the boot releases or open the boot when actually er opening the boot (ie the button above the numberplate).