You can see how he has used extra straws to create triangle shapes and strengthen the side of his tower. (science-sparks.com)
In essence the three rocks create a triangle with the height of the middle rock touching the line between the one on the left and the one on the right. (cbgallerygroup.com)
If possible, create a triangle using your focal point as the tip at the top of the triangle and decorating out from there. (suburbansimplicity.com)