Embodiments of three-dimensional (3D) memory devices and methods for forming the same are disclosed. In an example, a 3D memory device includes a substrate, a memory stack disposed on the substrate and including a plurality of interleaved conductive layers and dielectric layers, and a plurality of channel structures each extending vertically through the memory stack and having a plurality of protruding portions abutting the conductive layers and a plurality of normal portions abutting the dielectric layers. Each of the plurality of channel structures includes a blocking layer along a sidewall of the channel structure, and a storage layer over the blocking layer. The storage layer includes a plurality of charge trapping structures in the protruding portions of the channel structure, and a plurality of protecting structures in the normal portions of the channel structure and connecting the plurality of charge trapping structures.