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 Sigma 75 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. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.