A three-dimensional (3D) memory device includes a memory stack including conductive layers and dielectric layers interleaving the conductive layers, and a channel structure extending through the memory stack along a vertical direction. The channel structure has a plurality of protruding portions protruding along a lateral direction and facing the conductive layers, respectively, and a plurality of normal portions facing the dielectric layers, respectively, without protruding along the lateral direction. The channel structure includes a plurality of blocking structures in the protruding portions, respectively, and a plurality of storage structures in the protruding portions and over the plurality of blocking structures, respectively. A vertical dimension of each of the blocking structures is nominally the same as a vertical dimension of a respective one of the storage structures over the blocking structure.