A three-dimensional (3D) memory device includes a core array region and a staircase region adjacent to the core array region. The core array region includes a memory stack having a plurality of conductor layers and a plurality of dielectric layers stacked alternatingly, a first semiconductor layer disposed over the memory stack, and a channel structure extending through the memory stack and the first semiconductor layer. The staircase region includes a staircase structure, a supporting structure disposed over the staircase structure, and a plurality of contacts contacting the plurality of conductor layers in the staircase structure. The first semiconductor layer overlaps the core array region in a plan view of the 3D memory device and the supporting structure overlaps the staircase region in the plan view of the 3D memory device.