Self-assembly of symmetric ABC three-arm star copolymers confined in cylindrical nanopores is investigated by means of a lattice Monte Carlo simulation method. The dependence of morphologies on the degree of confinement and preference of pore surface is studied systematically. For the symmetric ABC three-arm star copolymers which form polygonal cylinder structures with periodic spacing L-0 in bulk, various novel structures are observed inside the nanopores. In the nanopores with a neutral surface, we find a minimum diameter value (D-min approximate to L-0) under which helical arranged droplets are formed; otherwise, parallel polygonal cylinder structures are identified. By adjusting the preference between component A and the pore surfaces, a number of novel structures such as A cylinder + BC single-strand helix and complex multilayer double helices are identified. Additionally, the confinement-induced morphology transition is interpreted by the frustration parameter D/L-0.