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Neutron diffraction and magnetic susceptibility studies show that orthorhombic single-crystals of the topological semimetals Sr(Mn_0.9Cu_0.1)Sb₂ and Sr(Mn_0.9Zn_0.1)Sb₂ undergo three-dimensional C-type AFM ordering at a significantly lower T_N than that of the parent compound SrMnSb₂. Magnetization versus magnetic field (applied perpendicular to the MnSb planes) measurements below T_N exhibit dHvA oscillations for the Sr(Mn_0.9Zn_0.1)Sb₂, but not for the Sr(Mn_0.9Cu_0.1)Sb₂. This suggests that doping with Cu for Mn either substantially changes the electronic band structure; or that the Cu sites are strong scatterers of carriers, which significantly shortens the mean free path, and diminishes the quantum oscillations. DFT calculations including spin-orbit coupling find that doping with either Cu or Zn tunes the electronic bands near the Fermi level, resulting in different band topologies and semimetallicity. These calculations rationalize the experimental results by showing that the parent and Zn-doped compounds have coexistence of electron and hole pockets with an open Dirac cone around the Y-point, whereas the Cu-doped compound has dominant hole pockets around the Fermi level with a distorted Dirac cone.