The 'pressure gauge technique' recently developed for the quantitative assessment of the mass of solid particles suspended at any agitation speed is extended to the case of three-phase stirred tanks. As a result, curves of the fraction of suspended solids at various gas flow rates, versus agitator speed, are presented. The difficulties involved in the extension of the technique to three phase systems are addressed and discussed. The experimental results show that the presence of the gas phase causes a significant increase of the agitation speed required to attain complete suspension of solids and lowers the degree of suspension at all agitation speeds below it. The experimental data obtained are shown to be well fitted by the Weibull functions previously adopted for two-phase systems. Correlations for the estimation of the influence of particle size, particle concentration and gas flow rate on the suspension degree are presented and discussed. Finally, the fractional suspension dependence on particle size is found to be very different when very large particles are dealt with.