Sand holdup is one of the most important hydrodynamic parameters that is needed for performance estimation, design, operation and control of oil-gas-sand multiphase production and pipeline transportation systems. The performance of oil-gas-sand multiphase flow can be reliably evaluated by measuring the sand holdup in such oil-gas-sand multiphase production and pipeline transportation systems. In the present work, a local sand holdup has been measured under conditions analogous to the horizontal oil-gas-sand three-phase slug flow in pipelines. Accurate local sand particle holdup measurements were performed by the digital imaging technique. The results revealed the influence of operating conditions such as gas and liquid velocities and sand particle loading on the distribution of the local sand particle holdup in the horizontal air-water-sand multiphase slug flow pipe. Explanations for the observed trends are provided, shedding light on the general structures and mechanisms of the distribution of the local sand holdup in a horizontal oil-gas-sand three-phase slug flow. Such information on the horizontal air-water-sand three-phase slug flow mechanisms are essential to advance the mechanistic approach for predicting local sand holdup distribution and the subsequent effect on sand deposition during multiphase petroleum production and transfer operations.