Self-assembly of block copolymers in three-dimensional (3D) soft confinement has become a versatile method for the preparation of functional microparticles. While the morphological behavior of AB diblock copolymers in confinement is fairly well understood, only sporadic examples exist for ABC triblock terpolymers. Considering the more complex morphological space of terpolymers, a better understanding of their behavior in confinement would be desirable to gain access to a large variety of multicompartment microparticles (MMs). Here, we report on the 3D confinement self-assembly of MMs from a library of polystyrene-block- polybutadiene-block-poly(tert-butyl methacrylate) (PS-b-PB-b-PT or SBT) triblock terpolymers with widely different block volume fractions (phi), which can be mapped into a ternary microphase diagram. Equal-sized end blocks (phi(S) phi(T)) result in MMs with lamella-lamella (LL) morphology and a tulip-bulb or pinecone shape, irrespective of phi B. For asymmetric end blocks (phi(S) > phi(T)) at low phi(B), prolate ellipsoids develop a hexagonally packed core-shell ring (HCR) morphology, which changes with increasing phi(B) to a transition structure between HCR and a concentric sphere-in-lamella (CSL) morphology. Further increasing phi(B) to phi(S) approximate to phi(B) at low phi(T), the structure transforms entirely to CSL in an onion-like shape. Selective cross-linking of the PB middle block and disassembly of the MMs give access to a variety of nanostructures such as core-shell rings, Janus nanobowls, and nanocups.