A series of tetranuclear [Cu3Ln] complexes, [Cu3Gd(L)3(NO3)2(H2O)3](NO3)·H2O (1), [Cu3Tb(L)3(NO3)2(H2O)3](NO3) (2) and [Cu3Dy(L)3(NO3)3(H2O)2]·1.5(H2O) (3), were synthesized by a one-pot reaction using a simple tetraketone-type ligand (H2L = (3Z,5Z)-4,5-dihydroxy-3,5-octadiene-2,7-dione). X-ray structural analyses revealed that each complex has a planar tetranuclear core of [Cu3Ln] (Ln = Gd, Tb, and Dy), in which the Ln ion is accommodated in the centre of a Cu3O6 metallocycle. A cryomagnetic study revealed that all complexes show intramolecular ferromagnetic interactions between Cu(II) and Ln(III) ions. The [Cu3Gd] complex (1) has an ST = 5 spin ground state and shows a magneto-caloric effect with a maximum magnetic entropy change (-ΔSm) of 16.4 J kg-1 K-1 (5 T, 2.4 K). On the other hand, the [Cu3Tb] complex (2) shows a slow magnetic relaxation behavior under a zero magnetic field. The analysis of an Arrhenius plot reveals that the effective energy barrier of spin reversal is 13.1 K. The [Cu3Dy] complex (3) also shows a slow magnetic relaxation under 1300 Oe dc magnetic field with an effective energy barrier of 6.82 K.