Sand-laden cavitating flows involve the evolution of solid particles, discrete bubbles, and macroscopic cavity structures across a wide range of temporal and spatial scales, posing a formidable challenge for accurate simulation. To simulate the complicated multiphase flow, a multiscale gas-liquid-solid modelling method in OpenFOAM has been developed in this work. The dynamics of particles and bubbles are resolved in the Lagrangian framework, whereas the cavity volume is captured in the Eulerian framework, while a two-way coupling algorithm is established to bridge these scales. The new solver is validated using available experimental data. Analyses of pure-water and sand-laden unsteady cavitating flows with three particle sizes over a Clark-Y hydrofoil indicate that particles promote cavitation and increase the cavity shedding frequency. The reduction in particle velocity near the wall increases the local fluid velocity, thereby reducing the adverse pressure gradient and weakening the re-entrant jet along the hydrofoil suction surface. The particle streamwise velocity distributions for all three particle sizes exhibit a Gamma distribution, with larger particles concentrating the streamwise velocity distribution. Particles promote the generation of discrete bubbles in the cloud cavitation, and increase the overall bubble size. Although the presence of particles does not modify the theoretical -10/3 power law in the large bubble regime, it increases the Hinze scale by weakening the turbulent dissipation rate and thereby suppresses the continuous breakup cascade from large to small bubbles. As a result, the power law of small bubbles deviates more strongly from the theoretical -4/3 scaling. These findings provide new insight into the instability mechanism of sand-laden cavitating flows.