Numerical results from a 3D diffraction model are presented where a Cycloidal Wave Energy Converter (CycWEC) is interacting with an incoming straight crested Airy Wave. The diffraction model was developed in response to experimental observations from 1:10 scale experiments which were conducted in the Texas A&M Offshore Technology Research center wave basin. These experiments were the first investigations involving a CycWEC where three dimensional wave diffraction effects were present due to the fact that the span of the CycWEC was much smaller than the width of the basin. The diffraction model predicted the observed surface wave patterns in the experiment well, and showed that diffraction induced wave focusing increased the recoverable wave power beyond the 2D predictions for small CycWEC spans, while approaching the 2D limit for very large spans. The numerical model was subsequently used to estimate the sensitivity of the CycWEC to misalignment between the incoming waves and the WEC shaft. The loss in efficiency was found to be strongly dependent on the ratio between WEC span and incoming wavelength, where short spans (on the order of one wave length or less) which are realistic for actual ocean deployment showed only minor reductions in efficiency, while very long spans were found to be more sensitive to misalignment.