Channel-type segregation is one of the defects in aluminum casting technology, particularly in the direct chill (DC) process, having a significant influence on the cast quality. Nevertheless, the formation mechanism of three-dimensional (3D) channel-type segregations remains poorly understood. In order to clarify the formation mechanism of defects of this type, we conducted a 3D numerical simulation of the DC casting process of an Al-Mg alloy billet considering the melt flow, heat and mass transfer, solidification, and the motion of the solidified ingot, coupled with the alloy phase diagram. The simulation results showed that the channel-type segregations have strip-patterns and formed easily at high casting speeds, being localized at a distance of a half-radius from the billet centerline. We also compared two-dimensional axisymmetric and 3D simulations, and the results indicated that a two-dimensional axisymmetric simulation is incapable of properly predicting the behavior of channel-type segregations.