The switch isn't making python3 available; both python2 and python3 have been available for users for a long time. The switch means having the system tools rely on python3 rather than on python2, such that you can successfully install, boot, and configure a Fedora install without python2. This involved porting all the system code that uses python.
I suspect Archlinux's job was easy, because they had fewer system tools that utilized python. For example, Pacman (Arch's package manager) is written in C, whereas Yum (Fedora's package manager) is written in Python. Fedora also has other components that use python (the firewall frontend, some build/install/setup scripts), and "switching" to python3 involves porting over all of that.
I suspect Archlinux's job was easy, because they had fewer system tools that utilized python. For example, Pacman (Arch's package manager) is written in C, whereas Yum (Fedora's package manager) is written in Python. Fedora also has other components that use python (the firewall frontend, some build/install/setup scripts), and "switching" to python3 involves porting over all of that.