We present a three-dimensional, fully quantum study of the diffractive scattering of He from isolated CO adsorbates on Pt(111). The nonspecular scattered He distributions are dominated by quantum interference effects such as Fraunhofer diffraction and rainbows. The determination of the final scattering distributions hence requires an exact treatment of the translational motion of the helium atom. Our wave packet simulations are free of dynamical approximations in the gas phase and provide the whole diffractive scattering distributions. One important motivation of this work was to assess the adequacy of a reduced dimensionality treatment to derive the geometrical parameters of the adsorption. It is found that, even if the interaction is characterized by azimuthal symmetry, a two-dimensional Fourier treatment is not sufficient to give reasonable estimates of both the size of the adsorbate and its position above the surface. We further investigate the scattering distributions as a function of the collision energy and discuss the appropriateness of a pure Fraunhofer model to interpret the results and derive geometrical parameters.