Ptychography is a new kind of lens-less imaging technology. What restricts the technique is the assumption of a multiplicative interaction between the illuminating coherent beam and the specimen, i.e., and the ptychography cannot be applied to samples no thicker than a few tens of micrometers in the case of visible-light imaging at micron-scale resolution. In the present work, we split a sample into axial sections, thereby realize three-dimensional ptychographic imaging of thick samples at the millimeter level in a series of computer simulations and optical experiments. Our simulation results reveal that by using single wavelength we cannot achieve good-quality images of thick samples. Thus it is necessary to introduce more wavelengths for illumination. With increasing the number of wavelengths, the imaging quality of three-dimensional thick samples can be enhanced continually. Then we make further study on the relationship between the imaging quality and the magnitude of wavelength in optical experiments by using two groups of samples with different thickness values. The results demonstrate that our experimental results are highly consistent with simulations. For our concrete configuration in this paper, the best results of imaging and separation may be obtained for the case of tri-wavelength. At the same time we make a reasonable explanation for the phenomenon of fold-over in the experiment. Our results are important and meaningful for the practical utilizing of three-dimensional ptychography of thick samples.