Xcode 9.3 Release was released on 29 Mar 2018 and requires macOS 10.13.2+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_9.3/Xcode_9.3.xip.
Release notes are located at https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-DontLinkElementID_1.
The SDKs it contains are:
- iOS 11.3 (Build 15E217)
- macOS 10.13.4 (Build 17E189)
- tvOS 11.3 (Build 15L211)
- watchOS 4.3 (Build 15T212)
The compilers it contains are:
- clang 9.1.0 (Build 902.0.39.1)
- swift 4.1 (Build 902.0.48)