Three-component colloidal hybrid nano-particles, which are central to a diverse array of applications, are typically synthesized using successive seeded growth steps, which are additive in nature and driven by surface chemistry considerations and material-specific preferences for nucleation and growth. Here, we describe a new nanoparticle insertion reaction for transforming heterodimers into heterotrimers, which is based on a supersaturation precipitation pathway that shifts the driving force for heterotrimer formation away from surface-driven nucleation and growth. To demonstrate the concept, a Ge segment is inserted between the Au and Fe3O4 domains of Au Fe3O4 heterodimers to form Au Ge Fe3O4 heterotrimers. Microscopic investigations reveal important mechanistic insights, including identification of a proposed Au Ge Au Fe3O4 intermediate. The process can be modified to access the analogous addition product Ge Au Fe3O4, allowing tuning between two distinct heterotrimer isomers with different configurations.