Xcode 3.0 Release was released on 25 Oct 2007 and requires macOS 10.5+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/xcode_3.0/xcode_3.0.dmg.
Release notes are located at https://download.developer.apple.com/Developer_Tools/xcode_3.0/xcode_3.0_release_notes.pdf.
The SDKs it contains are:
- iOS 2.0
- macOS 10.3
- macOS 10.4
- macOS 10.5