Embodiments of structure and methods for forming a three-dimensional (3D) memory device are provided. In an example, a 3D memory device includes a stack structure and at least one source structure extending vertically and laterally and dividing the stack structure into a plurality of block regions. The stack structure may include a plurality of conductor layers and a plurality of insulating layers interleaved over a substrate. The at least one source structure includes at least one support structure extending along the vertical direction to the substrate, the at least one support structure being in contact with at least a sidewall of the respective source structure.