With three layers the pants is extremely stiff, while the double layered canvas is more like a leather pants in stiffness. There are usually a significant difference in price between double and triple canvas pants. Only the experienced shooter can draw the full benefits from an extremely stiff pants as the pants will be so stiff that it will alter the position of a shooter unless he is used to his position. Therefore beginners should go for the cheaper double canvas pants and later upgrade to the stiffer third layer pants when he is experienced and comfortable with all the positions. There are two major materials that are used for pants and pants - leather and canvas.
Leather is just normal leather that is usually lined with canvas.
Canvas is a coarse cotton fabric that is made in a way to make it as stiff as possible. The leather pants is better suited for outdoor shooting as it doesn't get as affected by moist, while the canvas pants is much stiffer and give better support and stays stiff for a longer time . There is a hype among new shooter around the world to get new equipment when they start out. I believe that is a waste of money. A beginner doesn't know what is good for them and doesn't have the shooting skills.