Active matter consisting of agents that can move on their own is an example of non-equilibrium systems. Study of active matter has proven to be useful towards understanding the physics behind the cell growth, tissue regeneration, cancer invasion etc. Recent studies by Wu et al. (2017) on confined microtubule suspensions, a model system for active matter showed the surprising result that aspect ratio of a 3D channel is the crucial parameter that determines the type of self-sustained active flow, irrespective of the size of the channel. In order to elucidate this inherently 3D phenomenon, we use a continuum approach based model that accounts for active nematohydrodyanmics to study the three dimensionally confined active matter. Not only we reproduce the results of the experiments that aspect ratio of the channel plays major role in obtaining either a net or a no net flow state in the channel, we also explain the phenomena that dictates the type of active flow in a 3D channel. Our results also reveal the crucial role of flow aligning parameter, a material property of the active nematics in determining the flow state in a channel.