Is /usr/bin/python3 provided with macOS Catalina?
Kinda. A clean installation of Catalina includes a /usr/bin/python3 binary, but it's a stub for installing the command line developer tools, which includes Python 3.



If/when the command line developer tools are installed, the /usr/bin/python3 stub will run the actual python3 binary, but a clean install will just have the stub.

The Python 2 binaries (/usr/bin/python /usr/bin/python2) are still present on a default installation, but they are deprecated and the warning message says they will be removed in future versions (presumably 10.16).