To make a triple chest, simply stand next to the double chest you want to make a triple chest and use the command provided. Set the "X Y Z" to the X, Y, and Z coordinates that make it next to the double chest. This makes a invisible chest to the left. Opening the left or right chests will have a normal double chest interface, but if you open the middle you will see a graphically glitched and fully functional triple chest. Use this trick to give yourself extra space to store items.
(NOTE: This command does work in versions older than release 1.12, but I do not know all of the versions that this command works with.)