The "three elements" in the form of this painting method refer to shape, color and space.

1. Shape refers to the proportional form of an object. In sketching, form expresses the shape and proportion of objects through lines and the relationship between light and shade. Painters need to use changes in thickness, length, curvature, light and shade of lines to express the three-dimensional and spatial sense of objects.

2. Color refers to the light and dark forms of objects. In sketching, color expresses the relationship between light and dark and the three-dimensional sense of objects through changes in the weight, depth, virtuality and solidity of pencils. Painters need to express the color and texture of objects by observing and analyzing the impact of light sources on objects and the different ways in which objects reflect light.

3. Space refers to the perspective form and parallax effect of objects. In sketching, space uses perspective and parallax to express the position and distance relationship of objects in space. The painter needs to use the perspective of lines and changes in virtuality and reality to express the sense of space and distance of objects.