The flow field and void fraction distribution in an aerated tank of 0.45 m diameter with a three-impeller agitator have been numerically simulated and validated against prior experimental data. An asymmetric deep hollow-blade disk turbine (BT-6) was used as bottom impeller to disperse the incoming gas, with two uppumping, four-blade, Maxflo (MFu) hydrofoils above to ensure effective axial mixing from top to bottom of the tank. The standard Fulerian-Eulerian formulation of the k-e turbulence model with multiple frames of reference (MFR) was used in the simulation. A population balance model (PBM) combined with a multiple size group (MUSIG) model has been implemented using the commercial CFX code. Bubble breakup and coalescence have been modeled fundamentally using isotropic turbulence theory. An alternative approach was provided by an Euler-Euler computation assuming a constant single average bubble diameter (SABD), set to 4 mm. These SABD results were compared with those predicted by the MUSIG model and also with experiment. The liquid and gas flow fields, gas void fraction distribution, average bubble diameter, and spatial distribution of bubbles of differing diameter were computed and partially compared with available experimental data. The gas void fraction distribution is strongly affected and controlled by the flow field. There is a region with very high local gas holdup near the tank wall, just above the level of the top impeller. This was correctly predicted by the MUSIG model in good agreement with the experimental results. The results when using the assumption of a single average bubble diameter did not fit the experimental data quite as well.