Wire the IN end of each of your pebble pixel LED strips to the Sparkle Motion screw terminal parts as shown: 

• G wire to -
• Data wire to 19, 21 and 22
• Power wire to +

These strands are not marked in any way and it can be hard to tell which wire is which, and which end is which. They can vary by lot as well. You may need to do some testing before you get all the wires correctly installed.

There is a fourth pin output as well using the solder pads below the screw terminal. If you want to add a fourth strip, use pin 18. See the full pinout diagram here.