The microdomain structure of a model three-component, three-arm, star-shaped copolymer consisting of polystyrene (PS), poly(dimethylsiloxane) (PDMS) and poly(tert-butyl methacrylate) (PTBMA), each of them having nearly the same weight fraction, was investigated by means of differential scanning calorimetry (d.s.c.), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). D.s.c. results exhibiting the glass transition of PS and PTBMA and the crystallization and melting of PDMS strongly suggest the microphase separation of the three components into three microdomains. The microdomain structure is considered to be extremely complicated because the chemical junction points of the three constituent polymers must be confined on the lines where three kinds of interfaces meet. TEM and SAXS results strongly support the existence of a very regular microdomain structure with three-fold symmetry. Each of the three components possibly forms a three-dimensionally continuous network domain resulting in an ordered tricontinuous microdomain structure.