Metal additive manufacturing has enabled great diversity and design freedom in heat exchanger design. However, these benefits cannot be adequately realised without optimizing its ancillary components as well, specifically the inlet and outlet manifolds. Optimizing the manifolds can significantly improve the flow distribution inside the heat exchanger core. This in turn will improve the heat exchange performance and reduce flow obstruction. More importantly, it can reduce the size of the setup, which makes for much cheaper and faster manufacturing. Literature on optimization of flows is quite vast and extensive, but experimental validation is lacking. It is not well understood whether 2D optimized parts hold up to 3D validation, and whether it differs for laminar and turbulent regimes. In this work, a manifold is numerically optimized in 2D and 3D for various flow regimes including both laminar and turbulent flow. The ideal set of assumptions for each specific case is then prescribed based on the results. Validation is performed for some of the geometries after manufacturing them via 3D-printing.