Coupled grain boundary motion was simulated in a three-dimensional nanocrystalline Al grain boundary network using molecular dynamics. It is shown that, in spite of the triple junction constraints, a symmetrical Σ75 tilt boundary can migrate during the microplastic regime with the same coupling factor as when simulated in a bicrystal configuration. After reaching the full plastic regime, dislocations start coming into play, changing the grain boundary structure and hindering further coupled motion.