We present results for the electronic and atomic structures of different size molecular clusters containing sulfuric acid, water, and ammonia molecules. The electronic structure calculations are performed using the DMol3 quantum chemical program packages and the PW91 density functional. We report implications of our results for atmospheric cluster size distributions. Our calculations indicate that ammonia is bonded to the cluster structures much more strongly than water. The presence of ammonia also strengthens the binding of sulfuric acid molecules to the clusters, and significantly increases the formation of clusters containing two sulfuric acid molecules and 0--2 water molecules. However, owing to the low overall concentration of two-acid clusters, the fraction of ammonia-containing clusters in the calculated atmospheric cluster size distribution was still very low.