Xcode 9.3 Beta 1 was released on 24 Jan 2018 and requires macOS 10.13.2+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_9.3_beta/Xcode_9.3_beta.xip.
The SDKs it contains are:
- iOS 15E5167d
- macOS 17E139g
- tvOS 15L5164c
- watchOS 15T5165c
The compilers it contains are:
- clang 9.1.0 (Build 902.0.30)
- swift 4.1 (Build 902.0.34)