In this article the three-dimensional behavior of constitutive models containing fractional order time derivatives in their strain and stress operators is investigated. Assuming isotropic viscoelastic behavior, it is shown that when the material isincompressible, then the one-dimensional constitutive law calibrated either from shear or elongation tests can be directly extended in three dimensions, and the order of fractional differentiation is the same in all deformation patterns. When the material isviscoelastically compressible, the constitutive law in elongation involves additional orders of fractional differentiation that do not appear in the constitutive law in shear. In the special case where the material iselastically compressible, the constitutive laws during elongation and shear are different; however the order of fractional differentiation remains the same. It is shown that for anelastically compressiblematerial, thefour-parameter fractional solid—the rubbery, transition, and glassy model, which has been used extensively to approximate theelongationbehavior of various polymers, can be constructed from thethree-parameter fractional Kelvin—the rubbery transition model inshearand theelastic bulk modulusof the material. Some of the analytical results obtained herein with operational calculus are in agreement with experimental observations reported in the literature. Results on the viscoelastic Poisson behavior of materials described with the fractional solid model are presented and it is shown that at early times the Poisson function reaches negative values.