Thanks for the report but I am unable to reproduce the behavior you describe on another v10.14.5 system using any of the current python.org 3.7.3 macOS python binary, a built-from-source 3.7.3+, or a MacPorts 3.7.3 with either libedit or GNU readline.  First, I would check to check what the command "python3" really does.  In a new terminal window, I would expect something like:

$ type python3
python3 is /usr/local/bin/python3

Also check for any Python-related environment variables:

$ env | grep PYTHON

Since you appear to be using a Homebrew-installed python3.7, I would check the Homebrew forums and issue tracker for similar reports. You could also try reinstalling the Homebrew python3.