Ensure your base OS is the 64-bit SP1 version.
In 2016, Microsoft released what is effectively "Service Pack 2" without the name—the (KB3125574). This single package includes nearly every security and non-security update released between SP1 (2011) and April 2016. How to get it officially: windows 7 service pack 3 download 64-bit offline iso
Here is a technical comparison to clarify what you are actually downloading. Ensure your base OS is the 64-bit SP1 version
If you already have Windows 7 SP1 installed, you can manually download the "Convenience Rollup" to act as your SP3. KB3125574 on the Microsoft Update Catalog. Version: Choose the "X64-based systems" version for 64-bit. windows 7 service pack 3 download 64-bit offline iso