Embodiments of structure and methods for forming a three-dimensional (3D) memory device are provided. In an example, a 3D memory device includes a memory stack over a substrate, a plurality of channel structures, a source structure, and a support structure. The memory stack includes interleaved a plurality of conductor layers and a plurality of insulating layers. The plurality of channel structures extend vertically in the memory stack. The source structure includes a plurality of source portions and extending in the memory stack. The support structure is between adjacent ones of the source portions and has a plurality of interleaved conductor portions and insulating portions. A top one of the conductor portions is in contact with a top one of the conductor layers. Adjacent ones of the source portions are conductively connected to one another.