Effective time series analysis techniques in time and frequency domains were applied to characterize three-phase fluidization under a wide range of gas and liquid velocities. The experiments were carried out in a laboratory scale fluidized bed, operated under ambient conditions. Standard deviation of pressure fluctuations successfully detected four distinct regimes, namely compacted bed, agitated bed, coalesced and discrete bubble regimes (or discrete and dispersed bubble regimes at extremely low gas velocities). A minimum in skewness and average cycle frequency and a maximum in flatness indicated a minimum deviation from larger structures of the bed. A transition between macro and finer structures occurred during agitated bed regime and accordingly, minimum liquid fluidization velocity was perceived by skewness, flatness, and average cycle frequency analyses. The results showed that at very low liquid velocities in the compacted bed regime, the power spectrum at lower frequencies slightly increased with increasing liquid velocity. This results in the consecutive appearance of single bubbles of low frequencies. With further increase in the liquid velocity at agitated bed regime the power spectrum at lower frequencies gradually decreased. At higher liquid velocities near U-Lmf, the peak dominant frequency is transmitted from low frequency ranges of 2Hz to the higher frequency of about 10Hz.