A three-phase foam equation of state has been developed to complete a previous equation defined for aqueous foams by other authors. An original compression system has been used to define a pressure-specific volume relationship taking into account the presence of a co-volume in the three-phase foams. The results showed that three-phase foams are governed by an equation depending on pressure, temperature, foam specific volume, foam specific incompressible volume and moles of gas. This relation, P(V-s - V-si) = NgRT, will be, in addition way, a function of the solid content and the expansion ratio of the foam. Also, it has been found that the theoretical model of the three-phase foam equation coincides well with the experimental results, with an error less than 5%.