Since there is no SP3, the "latest" version you can download officially is Windows 7 with Service Pack 1 integrated.
I notice you're asking about a "Windows 7 Service Pack 3 ISO file." However, for accuracy: . The final service pack for Windows 7 is Service Pack 1 (SP1) , released in 2011. Any third-party site claiming to offer "Windows 7 SP3" is likely distributing unofficial, modified, or malicious software. windows 7 service pack 3 iso file download
— Required for modern code signing.
Install Windows 7 SP1. Do not connect to the internet yet. Since there is no SP3, the "latest" version