The magnetic properties of the trinuclear complexes M2UL (MII = Co, Ni, Cu; L = Schiff base), exhibiting the 3d-5f-3d magnetic core configuration, have been investigated theoretically using scalar relativistic DFT computations combined with the broken symmetry (BS) approach. The calculated coupling JM-U and JM-M constants agree with the observed antiferromagnetic character of the Co2UL and Ni2UL complexes, and the ferromagnetic character in the case of the Cu2UL one. The M-U and M-Ob-U structural parameters, as well as the electronic factors driving the superexchange couplings are discussed. The bond orders and the magnetic molecular orbital analyses reveal that the U(5f) covalent contribution to the bonding within the M-O-U coordination is more important in the Co2UL and Ni2UL complexes than in the Cu2UL congener, thus favouring AF coupling between the transition metal and the uranium magnetic centers. The computed ZnMUL7 and M2ThL7 analogous, where the CoII (3d7), NiII (3d8) and UIV(5f2) paramagnetic ions are replaced by the diamagnetic ZnII(3d10) and ThIV(5f0) diamagnetic ones, respectively. The Natural Populations Analyses confirm the crucial role of spin delocalization that is at work in favour of the AF vs. Ferro magnetic character of the M-U-M (M = Ni, Co) and Cu-U-Cu coordination, respectively.