In this work, we investigate the interface trap (IFT) variation effect, one of the most important reliability issues, in 5-nm node gate-all-around (GAA) single and 3D stacked nanowire field-effect transistors (NWFETs). Comparing the IFT variation effect, the 3D stacked NWFET was found to have stronger immunity compared to a single NWFET. However, the 3D stacked NWFET is significantly affected by the variation of each stack due to the process variation effect (PVE). For this reason, the goals of this paper were to reach a comprehensive understanding of the IFT variation effect and to provide an accurate guideline pertaining to the NW diameters in single NWFETs and 3D stacked NWFETs.