Nano beam electron diffraction is a powerful technique used for strain determination in thin crystalline and amorphous samples, as it combines high spatial resolution, accuracy, and precision. The influence of three-dimensional strain fields in samples, which are mostly present due to complex strain states or effects of free surface relaxations, is still unknown today. This study aims to increase the understanding of the averaging of the Bragg angle and the impact of such three-dimensional strain fields on the measured strain. To analyze the effect of a three-dimensional strain field on the average diffraction angle and strain evaluation, a combination of finite element and diffraction simulations was carried out. Specimen were modeled and deformed using finite element analysis. The atom positions were then interpolated into the finite element mesh and an electron diffraction simulation was executed. The obtained diffraction images were then evaluated using the square-root magnitude weighted phase correlation method. The resulting strain values were compared to the actual strain in the samples obtained from the finite element models. The simulations showed that the measured strains are in good agreement with the average strain along the microscope optical axis. However, a strong dependency of the measured strain from the angular deviation could be found. For higher absolute strain values and gradients, the diffraction simulations led to distorted images, but the strain evaluation using the square-root magnitude weighted phase correlation still yielded good results, proving the immense stability of this technique.