The self-assembly of a carbonylpyridine-based heptadentate ligand with LnIII results in the formation of triangular trinuclear europium complexes, which exhibit interesting luminescent properties in the solid state and in solution. With a view to developing multimodal responsive systems, we report here the preparation and characterisation of analogous complexes with GdIII. The X-ray crystal structure of Gd3L23 indeed reveals the isostructurality with the EuIII complexes. A combination of 1HNMRD and variable temperature studies yields the parameters elucidating the exchange of coordinated water and relaxivity properties. Conveniently, the competitive spectrophotometric titrations with EDTA and NTA are used to determine the thermodynamic stability constants of the europium complexes in aqueous media. In addition, the exchange reaction with EDTA is monitored with NMR and fluorimetry. The interactions of the EuIII trinuclear complex with some potentially interfering ligands are qualitatively investigated by means of luminescence titrations.