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 having interleaved a plurality of conductor layers and a plurality of insulating layers, a plurality of channel structures extending in the memory stack, and a source structure extending in the memory stack. The source structure includes a plurality of source contacts each in a respective insulating structure. Two adjacent source contacts are conductively connected to one another by a connection layer, the connection layer includes a pair of first portions being over the two adjacent ones of the plurality of source contacts and a second portion between the pair of first portions. A support structure is between the two adjacent source contacts. The support structure includes a cut structure over interleaved a plurality of conductor portions and a plurality of insulating portions.