Clay is the thickest of the three.
Why
Clay: Particles are the smallest, typically less than 0.002 millimeters. Their tiny size lets them pack tightly, creating dense, sticky material.
Silt: Particles are slightly larger than clay, from 0.002 to 0.05 millimeters. Finer than sand but not as fine as clay, resulting in a less dense texture.
Sand: Particles are the largest, from 0.05 to 2 millimeters. Their size prevents tight packing, making sand looser.
Clay is thickest because its extremely fine particles pack densely together.