Embodiments of three-dimensional (3D) memory devices having a memory layer that confines electron transportation and methods for forming the same are disclosed. A method for forming a 3D memory device includes the following operations. First, an initial channel hole can be formed in a structure. The structure can include a staircase structure. The structure can include a plurality first layers and a plurality of second layers alternatingly arranged over a substrate. An offset can be formed between a side surface of each one of the plurality of first layers and a side surface of each one of the plurality of second layers on a sidewall of the initial channel hole to form a channel hole. A semiconductor channel can then be formed based on the channel hole. Further, a plurality of gate electrodes can be formed based on the plurality of second layers.