The validity of the computational method for particle-laden liquid flows, MICS (Multiphase Incompressible flow solver with Collocated grid System), was shown through the comparison with experimental results in terms of unsteady wakes behind particles and interaction between particles and surrounding flows. In this paper, MICS is improved to the computational method for gas-liquid-solid multiphase flows in order to deal with the particle movements in the liquid flow with large deformation of the free surface. MICS is based on the computational method for incompressible flows, in which the treatment for density and mass conservation has been improved in the collocated grid. The external forces for the particles are evaluated from the results of the fluid computations and a model of the distinct element method (DEM). The MICS was applied to the particle movements in a free-surface flow and its applicability is discussed.