An Eulerian-Lagrangian computational model for simulations of gas-liquid-solid flows in three-phase slurry reactors is developed. In this approach, the liquid flow is modeled using a volume-averaged system of governing equations, whereas motions of bubbles and particles are evaluated by Lagrangian trajectory analysis procedure. It is assumed that the bubbles remain spherical and their shape variations are neglected. The two-way interactions between bubble-liquid and particle-liquid are included in the analysis. The discrete phase equations include drag, lift, buoyancy, and virtual mass forces. Particle-particle interactions and bubble-bubble interactions are accounted for by the hard sphere model approach. The bubble coalescence is also included in the model. The predicted results are compared with the experimental data, and good agreement is obtained. The transient flow characteristics of the three-phase flow are studied and the effects of bubble size on variation of flow characteristics are discussed. The simulations show that the transient characteristics of the three-phase flow in a column are dominated by time-dependent staggered vortices. The bubble plume moves along the S-shape path and exhibits an oscillatory behavior. While particles are mainly located outside the vortices, some bubbles and particles are retained in the vortices. Bubble upward velocities are much larger than both liquid and particle velocities. In the lower part of the column, particle upward velocities are slightly smaller than the liquid velocities, while in the upper part of the column, particle upward velocities are slightly larger. The bubble size significantly affects the characteristics of the three-phase flows and flows with larger bubbles appear to evolve faster.