Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a substrate, a memory stack including interleaved conductive layers and dielectric layers above the substrate, a plurality of channel structures each extending vertically through the memory stack, a semiconductor layer above and in contact with the plurality of channel structures, a plurality of source contacts above the memory stack and in contact with the semiconductor layer, a plurality of contacts through the semiconductor layer, and a backside interconnect layer above the semiconductor layer including a source line mesh in a plan view. The plurality of source contacts are distributed below and in contact with the source line mesh. A first set of the plurality of contacts are distributed below and in contact with the source line mesh.