High or low, it's up to you

By using the sainsmart LCD Keypad Shield,we can control the intensity of the backlight with the Arduino digital pin D3.That is to say,the backlight will become on and off if you set the pin D3 high or low.It's up to you.
First of all,we can Vary Backlight Intensity:

Generally speaking,We choose pin which is capable of "PWM",to control the backlight. such as Arduino pin D3. Using PWM you can vary the intensity of the backlight by setting pin D3 to different levels between 0% and 100% using the analogWrite() function.
If you want to combine the sainsmart LCD Keypad Shield with another shield which also uses Arduino pin D3, you may want to change the backlight control over to another Arduino pin. Fortunately,the shield is designed to make this easy to do.
The pair of pads labelled "D3" is connected on the PCB by a tiny track which you can easily see, so the first step is to use a sharp knife to cut the track between the two pads.
Now that you've disconnected Arduino pin D3 from the backlight control, you can either leave the backlight control disconnected entirely (in which case it will be permanently on) or connect it to a different pin using one of the other pairs of jumpers.
Pick the pin you want to use to control the backlight, please remember that not all pins can do PWM,so you may only be able to turn the backlight on or off rather than vary its brightness, and connect those pads by using a drop of solder.