The complex Na₃[Cu₃(mal)₃(H₂O)]·8H₂O was obtained from evaporation of an aqueous solution containing Cu(OAc)₂, malic acid (HO₂CCH₂CHOHCO₂H) and NaOH and was characterised by X-ray diffraction on single crystal, X-band and high-field EPR spectroscopy (HF-EPR) and magnetic susceptibility measurements. The trinuclear complex [Cu₃(mal)₃(H₂O)]³⁻ is trapped in a three-dimensional network with sodium cations. The three copper atoms are connected by alkoxo bridges and form an almost isosceles triangle with Cu⋯Cu distances of 3.076(1), 3.504(1) and 3.513(1) Å. Two of the copper ions are also bridged by an extra aquo ligand. EPR spectroscopy combined with magnetic susceptibility measurements provide a powerful tool to resolve the electronic structure of the complex. The overall magnetic behaviour corresponds to an antiferromagnetically coupled triangular system. The 285 GHz-EPR spectrum (g = 2; 10.18 T) is characteristic of a spin state S = 1/2, with a rhombic anisotropy of [g]. This rhombic pattern allows us to propose that the electronic spin density is delocalised on the three copper ions.