Xcode 3.1.3 Release was released on 16 Jun 2009 and requires macOS 10.5.7+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/xcode_3.1.3_developer_tools/xcode313_2736_developerdvd.dmg.
Release notes are located at https://download.developer.apple.com/Developer_Tools/xcode_3.1.3_developer_tools/about_xcode_tools_3.1.3.pdf.
The SDKs it contains are:
- iOS 3.1.3