Unfortunately, there isn't a Windows 7 Service Pack 3 available. Microsoft released only two service packs for Windows 7:
Since an official "SP3 ISO" does not exist, users typically follow these steps to create a modern, updated offline installer: Windows 7 - Microsoft Lifecycle
Ensure your base OS is the 64-bit SP1 version.
This update prepares the Windows Update system to handle the massive rollup package.
While the Convenience Rollup secures your system up to 2016, Windows 7 has been entirely unsupported since January 2020. This means your system remains vulnerable to critical exploits discovered after that date (such as advanced remote code execution vulnerabilities) if it is connected to the internet.
Instead of searching for an unofficial SP3, you should download the official . This package functions exactly like a Service Pack, upgrading a clean Windows 7 SP1 installation with five years of patches in a single, offline installer. Prerequisites for Installation
Windows 7 Service Pack 3 Download 64-bit Offline ISO: What You Need to Know