It is shown that the van der Waals free energy of polydisperse fluids, as introduced previously [L. Bellier-Castella, H. Xu, and M. Baus, J. Chem. Phys. 113, 8337 (2000)], predicts that for certain thermodynamic states (e.g., low temperatures and large polydispersities) the ordinary two-phase coexistences become metastable relative to a fractionation of the system into three phases, reducing thereby the polydispersity of each of the coexisting phases.