Xcode 6.3 Release was released on 8 Apr 2015 and requires macOS 10.10+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_6.3/Xcode_6.3.dmg.
Release notes are located at https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-SW378.
The SDKs it contains are:
- iOS 12F69
- macOS 13F34
- macOS 14D125
The compilers it contains are:
- clang 6.1.0 (Build 602.0.49)
- swift 1.2 (Build 602.0.49.3)
- LLVM-GCC 3.6.0 (Build )