Janus particles have drawn considerable interest as colloidal surfactants, microswimmers, and building blocks for colloidal lattices. So far, research primarily focused on spherical Janus particles for which a number of fabrication methods are well established. Janus particles with geometric anisotropy offer shape-dependent properties in addition to surface anisotropy, but their synthesis is more challenging. Here, we report a variety of polymeric Janus nanoparticles synthesized from ABC triblock terpolymer microphases in microemulsion droplets. Evaporation-induced assembly of the ABC triblock terpolymers led to prolate microparticles with A/C lamellae stacked along the particle's major axis. By admixing a B homopolymer during microphase separation, the morphology of the B microphase was gradually tuned in between the A/C lamellae. Cross-linking of the B microdomain created Janus nanodiscs, where the topology could be controlled to unconventional inner structures. We follow the morphology evolution and rationalize their stability with theoretical considerations in the strong segregation limit and dissipative particle dynamics simulations.