Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a method for forming a 3D memory device is disclosed. A first polysilicon layer, a dielectric sacrificial layer, a second polysilicon layer, and a dielectric stack are sequentially formed above a substrate. A channel structure extending vertically through the dielectric stack, the second polysilicon layer, and the dielectric sacrificial, and into the first polysilicon layer is formed. An opening extending vertically through the dielectric stack and the second polysilicon layer, and extending vertically into or through the dielectric sacrificial layer to expose part of the dielectric sacrificial layer, and a polysilicon spacer along part of a sidewall of the opening are formed. The dielectric sacrificial layer is replaced, through the opening, with a third polysilicon layer between the first and second polysilicon layers.