The novel concept of trajectory length distribution (TLD), recently introduced by Villermaux (1996, Chem. Engng Sci. 51, 1939) for characterizing the macromixing of fluid elements in flow systems, is used to describe solids mixing in the fully developed zone (FDZ) of three-phase fluidized beds. Tests with monodispersed and binary mixtures of solids of different sizes and densities were performed in the dispersed and the coalesced bubble flow regimes. The Lagrangian trajectories of single traced particle were measured non-invasively via radioactive particle tracking (RPT). A macromixing index was derived from the experimental TLDs for describing solids mixing in both upward and downward sections of the FDZ. Residence-time distributions (RTDs) of the solid particles, moving upwards in the column core section and downwards in the sidewall section of the FDZ, were generated from the RPT-measured trajectories. Based on the observed physical features of the solids flow, these RTDs were modeled using a two-zone one-dimensional cross-flow multistage stirred reactors (CFMSR) model. Part of the CFMSR input parameters were obtained taking advantage of the exhaustive three-dimensional trajectories measured from RPT. The model is shown to compare well with the experimental RTDs in both ascending and descending sections.