The paper performs an investigation on multi-scale coherent structures that exist in different flow patterns of triple interacting plumes. Time-averaged and turbulent flow characteristics are first obtained by the two-dimensional (2-D) particle image velocimetry (PIV) technique. Proper orthogonal decomposition (POD) method is then applied to extract the dominant coherent structures. Energy contributions, POD modes, and POD coefficients are systematically explored to understand the spatial-temporal characteristics of the coherent structures. The first three POD modes are most important in the turbulent kinetic energy (TKE). Mode 1, as the most dominant mode, can contain about 10% TKE in an average sense. In addition, the first 12–14 POD modes can totally contribute 50% TKE. Hierarchical four-scale flow structures, independent of the flow patterns, are clearly identified when mode number increases, which are successively characterized by main flows (the 1st scale), larger-scale outer-side vortex rings (the 2nd scale), shear-layer vortex shedding (the 3rd scale), and smaller-scale homogeneous turbulent vortices (the 4th scale). Frequency spectra analysis confirms the co-existence of lower-frequency and higher-frequency periodic cycles in one certain POD mode. Dominant frequencies of the first three POD coefficient series (a1–a3) fall into the range of 0.025–0.187 Hz. Probability density analysis has identified negative/positive peak probability densities of the series a1. Cumulative probabilities of negative and positive coefficients are approximately equivalent during the sampling period.