New ternary and higher order inorganic materials are needed for a large variety of applications, yet their synthesis still represents a chemistry challenge. Herein, we focus on the synthesis of Cu-M-S nanocrystals (where M = V, Cr, Mn) via a wet-chemistry route and investigate their formation mechanisms. We reveal that the interplay between the copper precursor and the thiophilicity of the transition metal M is the key for the synthesis of pure phase Cu-M-S nanocrystals under the same reaction conditions. In particular, we observe that the interdiffusion kinetics of the intermediate species is crucial, and the extent of nucleation of the ternary product can be controlled by the copper precursor reactivity. The insights provided by this work contribute to open up new avenues toward the design of improved synthesis strategies to multinary nanocrystalline compounds.