Multicompartment micelles, as novel nanoscopic structures, have great potentialities in the fields of multifunctional nanoreactors and carriers. In this work, multicompartment micelle formation in an initially homogeneous dilute solution of ABC miktoarm star terpolymer is investigated with polymeric external potential dynamics. Apart from the hamburger micelle, toroidal micelle, raspberry micelle, worm micelle and laterally structured vesicle, which have been reported before, three novel morphologies are observed, including laterally structured vesicle with a core, spotted vesicle with a core and segmented cage-like micelle, by varying the concentration of copolymers, the volume fractions of three blocks, the solvophobicity of the solvophobic A and C blocks, and the solvophilicity of the solvophilic B block. The structural stability of the prolate vesicle with alternating ring-shape AC strips is demonstrated using Helfrich's membrane model combining with the strong segregation theory of block copolymers. In the dynamics of vesicle formation, two formation pathways of multicompartment vesicles are found: when the volume fraction of B block is smaller, the formation pathway of vesicles includes nucleation, coalescence and growth; when the volume fraction of B block is larger, the formation process of vesicles only includes nucleation and growth. The formation mechanisms of toroidal and cage-like micelles are also studied in this work. Our simulation results enrich the knowledge of the morphologies of multicompartment micelles and reveal the formation mechanism of complex multicompartment micelles of miktoarm star terpolymer in solution.