A 3D memory device includes a memory stack including a memory block. The memory block includes a first memory array structure, a staircase structure, a second memory array structure in a first lateral direction, and a plurality of strings in a second lateral direction. The staircase structure includes a staircase zone and a bridge structure adjacent to the staircase zone. The 3D memory device also includes a SSG cut structure. The SSG cut structure includes a first portion between a first string and a second string and extends in the bridge structure in the first lateral direction. The staircase zone includes a first staircase conductively connected to first memory cells in the first string through the bridge structure and a second staircase conductively connected to second memory cells in the second string in the first memory array structure through the bridge structure.