Firstly, to create 3-D chalk art you need to be a good artist. However, that's just for 2-D art. For 3-D you need to be able to produce shades. Our eyes use shadows to discern distance and perspective and location. If you can use that in artwork you ca make the image appear as if its in 3-D because it tricks your brain into thinking the objects are farther away and in different locations because of the shade, shadows, and perspective. You have to be in a specific location however for your brain to interpret theses shades correctly.
“Optical Illusions." Brain Games. N.p., n.d. Web. 16 Mar. 2015.