Mechanism of broad acoustic band_gap in the two_dimensional three_component composite is investigated by calculating acoustic transmission spectra and acoustic fields for different inserted steel rows embedded in epoxy resin matrix with or without rubbers as the coating material. By comparing the acoustic fields inside the two_ and three_component crystal slabs, one can conclude that the broad acoustic band_gap in the three_component crystal slab is formed by the coexistence of the Bragg reflection of a periodic structure and the localized resonance in the soft rubber. The former is evidence in higher frequency and the latter is dominant in lower frequency with the wavelength longer than that of the coating layer.