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: http://geomodelsolar.eu/_docs/papers/2014/Suri-Cebecauer--ASES-Solar2014--Satellite-Based-Solar-Resource-Data--Model-Validation-Statistics-Versus-User-Uncertainty.pdf
Information on accuracy of NREL TMY3 data can be seen here: http://www.nrel.gov/docs/fy08osti/43156.pdf