Stop looking for Service Pack 3. It does not exist. Your target is Service Pack 1 + KB3125574 .
While Windows 7 is still functional, it is no longer receiving security patches from Microsoft. If you are using a "Service Pack 3" style ISO, ensure you are using a robust third-party firewall and avoid using the system for sensitive tasks like online banking. windows 7 service pack 3 download 64-bit offline iso
Since the Windows Update servers for Windows 7 are often unreliable or slow, having the updates in an offline .msu file ensures success. Stop looking for Service Pack 3