There is no official Service Pack 3 (SP3) for Windows 7 . Microsoft only released one official service pack, Service Pack 1 (SP1)
If you are looking to update a 64-bit Windows 7 system to its most current official state, you should focus on Service Pack 1 and the subsequent "Convenience Rollup." 1. The Final Official Update: Service Pack 1 (SP1) windows 7 service pack 3 download 64-bit