A three-dimensional (3D) memory device having a plurality of vertical channel structures includes a first memory block, a second memory block, and a bit line. The first memory block includes first vertical channel structures extending in a vertical direction with respect to a surface of a substrate. The second memory block includes second vertical channel structures on the first vertical channel structures in the vertical direction and first and second string selection lines extending in a first horizontal direction and offset in the vertical direction. The bit line extends in the first horizontal direction between the first and second memory blocks and is shared by the first and second memory blocks. The second memory block may include first and second string selection transistors which are each connected to the bit line and the first string selection line and have different threshold voltages from each other.