We prove that the moduli spaces A 3 (D) of polarized abelian threefolds with polarizations of types D = (1, 1, 2), (1, 2, 2), (1, 1, 3) or (1, 3, 3) are unirational. The result is based on the study of families of simple coverings of elliptic curves of degree 2 or 3 and on the study of the corresponding period mappings associated with holomorphic differentials with trace 0. In particular we prove the unirationality of the Hurwitz space H 3,A (Y ) which parameterizes simply branched triple coverings of an elliptic curve Y with determinants of the Tschirnhausen modules isomorphic to A −1 .