A series of six isostructural heterometallic trinuclear oxalate-bridged complexes of the formula (TDbenz)₂(TsO)₂[M^II(H₂O)₂{(μ-ox)M^III(ox)₂}₂]·6H₂O·2CH₃OH (TDbenz = 1,3,5-tris[2-(1,3-diazolinium)]benzene; TsO = 4-methylbenzenesulfonate; ox = oxalate; M^III = Fe, M^II = Mn (1), Fe (2), Co (3); M^III = Cr, M^II = Mn (4), Fe (5), Co (6)) have been synthesized from (NH₄)₃[M^III(ox)₃]·3H₂O, the chloride salts of the divalent metal ions and the tosylate salt of 1,3,5-tris[2-(1,3-diazolinium)]benzene (trisamidinium). Whereas the crystal structures of compounds 2, 3, 4 and 5 have been investigated by single-crystal X-ray diffraction, the structures of 1 and 6 have been checked by X-ray powder diffraction. All six compounds are isostructural and crystallise in the P space group. The crystals are composed of discrete linear [M^II(H₂O)₂{(μ-ox)M^III(ox)₂}₂]⁴⁻ trinuclear bimetallic units, trisamidinium and tosylate ions and solvent molecules. The linear trinuclear unit is based on a central trans-diaquametal(II) entity connected to two [M^III(ox)₃]³⁻ (M^III = Cr^III, Fe^III) moieties through oxalate bridges. The divalent metal ions, surrounded by six oxygen atoms, adopt a distorted octahedral coordination geometry. The coordination sphere is composed of four oxygen atoms belonging to two oxalate ligands and two trans-coordinated water molecules. One of the oxalate ions is coordinated to the central metal centre whereas the other two oxalate ligands are non-bridging. In the crystal, intermolecular hydrogen bonds involving oxalate ligands, solvent molecules and the counter-ions form a complex 3D network. Variable-temperature magnetic susceptibility measurements indicate an antiferromagnetic interaction between the iron(III) and the metal(II) ions (J = −4.23, −6.73, −8.97 cm⁻¹ for 1, 2 and 3 respectively) whereas this interaction is ferromagnetic when iron(III) is replaced by chromium(III) (J = +1.21, +2.20, +3.63 cm⁻¹ for 4, 5 and 6 respectively). Moreover, the cobalt(II) derivatives exhibit high D values (D = 29.3 cm⁻¹ for 3 and D = 27.4 cm⁻¹ for 6).