Additively manufactured (AM) components have hierarchical and complex microstructures, which lead to distinct properties while used in applications. Among various AM techniques, selective laser melting was selected in the present study to fabricate cuboid-samples of 316L stainless steel. The samples were created by different laser scanning schemes which resulted in different melt pool geometries and, consequently, different microstructures and textures. In order to fully reveal the microstructures and their formation mechanisms, our large-volume 3D EBSD system, ELAVO 3D, was employed. This system combines fully automated mechanical serial polishing with EBSD-based orientation microscopy and enables detailed crystallographic investigation of volumes up to almost one cubic millimeter. We show that it is not only desired but essential to characterize AM microstructures in three dimensions. Grain morphologies, grain boundary character distribution, and in-grain crystallographic rotation were analyzed in detail with the aim to understand the nucleation mechanisms inside of the melt pools and the growth principles of the large and tree-like grain structures. Finally, the microstructures are correlated with their responses to impact toughness tests.