Xcode 12.3 RC 1 was released on 8 Dec 2020 and requires macOS 10.15.4+.
It can be downloaded from https://download.developer.apple.com/Developer_Tools/Xcode_12.3_Release_Candidate/Xcode_12.3_Release_Candidate.xip.
Release notes are located at https://developer.apple.com/documentation/xcode-release-notes/xcode-12_3-release-notes/.
The SDKs it contains are:
- iOS 14.3 (Build 18C61)
- macOS 11.1 (Build 20C63)
- tvOS 14.3 (Build 18K559)
- watchOS 7.2 (Build 18S561)
The compilers it contains are:
- clang 12.0.0 (Build 1200.0.32.28)
- swift 5.3.2 (Build 1200.0.45)