To enable backlight dimming using the CircuitCo LCD3 cape, you need to connect the backlight boost circuit EN line to EHRPWM1A, by removing R126 and adding a 0R resistor (or blob of solder) to the pads for R123.
Then, you can issue the command
echo 38 > /sys/class/backlight/backlight/brightness
to control backlight intensity.
