The extended site‐binding model, which explicitly separates intramolecular interactions (i.e., intermetallic and interligand) from the successive binding of metal ions to polytopic receptors, is used for unravelling the self‐assembly of trimetallic double‐stranded CuI and triple‐stranded EuIII helicates. A thorough analysis of the available stability constants systematically shows that negatively cooperative processes operate, in strong contrast with previous reports invoking either statistical behaviours or positive cooperativity. Our results also highlight the need for combining successive generations of complexes with common binding units, but with increasing metallic nuclearities, for rationalizing and programming multicomponent supramolecular assemblies.