Three-dimensional (3D) memory devices and methods for forming the 3D memory devices are provided. In one example, a 3D memory device includes a substrate, a memory stack including interleaved conductive layers and dielectric layers on the substrate, and a staircase structure on one side of the memory stack. The 3D memory device also includes a staircase contact in the staircase structure and a plurality of dummy source structures each extending vertically through the staircase structure. The plurality of dummy source structures surround the staircase contact.