The composition of meteorites and the surface of asteroids suggest that planetesimals of the early solar system have undergone partial melting and differentiation. The sepa- ration of the denser metal (Fe-FeS alloy) from the lighter silicate is the most important differentiation process. The melting is mainly induced by the heat produced through the decay of 26Al and 60Fe. The distribution of these heat sources inside the celestial body is not uniform. In fact, 26Al is a lithophile element following the migration of the silicate and 60Fe is a siderophile element following the metal. In modeling the differen- tiation of small bodies it is fundamental to include at least two fluid phases in addition to the solid matrix. This study presents a first time three-phase mixture model for the metal-silicate segregation in a compacting body. The theoretical model is developed fol- lowing the classical averaging approach. The governing equations are then implemented in a numerical model in 1-D spherical geometry. In presence of two fluids, these can exchange their position within the porous matrix even in absence of compaction. They also act a mutual viscous drag force, which results in small fractions of metal to ascend with the lighter silicate, and viceversa.