Embodiments of a three-dimensional (3D) memory device and fabrication method thereof are disclosed. The 3D memory device has an architecture with an increased number of bit lines. In an example, a stack structure is formed above a substrate. A plurality of memory strings each extending vertically through a memory region of the stack structure are formed. A plurality of bit lines are formed over the plurality of memory strings, such that at least one of the plurality of bit lines is electrically connected to a single one of the plurality of memory strings.