A three-dimensional (3D) memory device and a fabricating method for forming the same are disclosed. The 3D memory device can include an alternating conductor/dielectric layer stack disposed on a substrate, a first staircase structure and a second staircase structure formed in the alternating conductor/dielectric layer stack, a staircase bridge extending in a first direction and electrically connecting the first staircase structure and the second staircase structure, and a first bottom select gate segment covered or partially covered by the staircase bridge. The first bottom select gate segment can include an extended portion extending in a second direction different from the first direction.