The magnetic and crystal structures of the title materials have been investigated using high-resolution neutron and synchrotron x-ray powder diffraction. Ferromagnetic Ru dimers are observed in the spin-ordered ground states of all three materials, evidencing an intradimer double exchange interaction. Antiferromagnetic order between dimers breaks the degeneracy of the triangular lattice but no distortion of the B′=La and Y structures is observed. The magnetic ordering transition temperature increases with the radius of the B′ cation. An anomalous 120 K structural transition in Ba3NdRu2O9 is attributed to Jahn-Teller distortion, evidenced by large-amplitude Γ6 deformations of the NdO6 octahedra. Further lattice anomalies accompany magnetic ordering transitions at 24 and 18 K in Ba3NdRu2O9. Nd spins order ferromagnetically and a substantial magnetic anisotropy leads to large coercivity and 1/3-magnetization steps in the magnetic hysteresis loop.