This study examined the fluidization behavior of gas-liquid-solid magnetic fluidized beds to provide prerequisite knowledge for reactor design. Air, water, and iron shots (0.19, 0.39, and 0.93 mm in diameter) were used as gas, liquid, and solid phases, respectively. The radial gas holdup distribution and overall gas holdup in three-phase magnetic fluidized beds were measured. Flow regimes of three-phase magnetic fluidized beds with different particle sizes were also discussed. The radial gas holdup distribution was measured by using an optical fiber probe; the change of radial gas holdup distribution under different flow regimes was also studied.