The objective of this work was to investigate the relative contribution of the convective and dispersive mixing mechanisms to the overall solid phase mixing mechanism for three-phase fluidized bed reactors. Noninvasive Radioactive Particle Tracking (RPT) data were obtained at various operating conditions, reactor diameters and particle systems. The structural wake model was updated and consists of three sub-phases: the particle wake and downflow-emulsion phase following the convective mixing mechanism and the vortex-emulsion phase following the dispersive mixing mechanism.The particle velocity mean and STD increased with the superficial liquid and gas velocity as well as the reactor diameter for each particle phase. Therefore, the extent of mixing increased under these operating parameters. The particle phase's holdup and a new Mixing Mechanism Indicator (MMI), however, exhibited a much more complex trend. For the larger reactor (Dc = 0.292 m), the convective mixing mechanism dominated and at low superficial liquid velocity the contribution of the convective mixing mechanism increased with superficial gas velocity. This trend, however, was reversed when the superficial liquid velocity increased. For the smaller reactor (Dc = 0.10 m), the random movement of the solid dominated. Therefore, the extent of mixing and the mixing mechanism did not follow the same trend. The CFD and the mixing model have to follow both those hydrodynamic properties.Relations to estimate mean and STD particle velocity distribution as well as particle phase holdup were developed. Solid in the established region mainly followed the convective mixing mechanism. Random movement of the solid was mostly observed at the top of the bed, but it was also present in the established region and at the bed bottom. The volume of each region depended on the operating conditions, reactor diameter and particle system. Furthermore, the particle wake expulsion frequency range was similar to the wake shedding frequency found in literature. It is, thus, possible to assume that wake shedding was mainly responsible for the solid exchange between particle phases.