Using a multi-component metallic glass as a model, we advanced atomic electron tomography to determine its 3D atomic positions and chemical species with a precision of 21 picometer. We quantified the short-range order (SRO) and medium-range order (MRO) of the 3D atomic arrangement. We observed that some SRO structures connect with each other to form crystal-like networks and give rise to MRO. We identified four types of MRO - face-centred cubic, hexagonal close-packed, body-centered cubic and simple cubic - coexisting in the sample, showing translational but not orientational order. We quantified the size, shape, volume, and structural distortion of MRO with unprecedented detail. Looking forward, we anticipate this experiment will open the door to determining the 3D atomic coordinates of various amorphous solids, whose impact on non-crystalline solids may be comparable to the first 3D crystal structure solved by x-ray crystallography over a century ago.