NREL TMY3 data are available for only specific locations. Whereas SolarGIS is a satellite-derived irradiance database with continuous availability at spatial resolution of 250m x 250m. SolarGIS compares favourably against NREL TMY3 on several aspects including accuracy, resolution, and continuity.

Details of accuracy of SolarGIS data can be seen here:

Information on accuracy of NREL TMY3 data can be seen here: