“E: Unable to locate package python3-pip”

For Ubuntu 18.04 (Bionic Beaver), with Python 3.6.5, the python-pip package will install after:

sudo add-apt-repository universe
sudo apt-get update

Which is enabling the category universe

deb http://archive.ubuntu.com/ubuntu bionic main universe
deb http://archive.ubuntu.com/ubuntu bionic-security main universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main universe