Xcode 10.x.x cannot build a Swift 3.x iOS application.
Xcode 9.x.x is required to perform the conversion away from Swift 3.x.
Xcode 9.x.x is not compatible with Catalina 10.15.2.
I am unable to locate installers for previous versions of macOS. Additionally, the hardware is a 16-inch MacBook Pro which, if I understand correctly, cannot run Mojave.
What is the present-day best practice for performing a Swift 3.x conversion from the 16-inch MacBook Pro?