Steps to install ROS with python3.

ROS-Melodic-Python3

# Warning: don't try to run this as a script. It probably won't work.

# remove all things python (optional)
sudo apt-get remove python-*

# remove previous installations of ROS
sudo apt-get remove ros-*
sudo apt-get remove ros-melodic-*
sudo apt-get autoremove

# setup python3 dependencies
sudo apt update
sudo apt install -y python3 python3-dev python3-pip build-essential
sudo -H pip3 install -U rosdep rospkg rosinstall_generator rosinstall wstool vcstools catkin_tools catkin_pkg
# initializ catkin build environment
sudo rosdep init
rosdep update
cd ~
mkdir ros_catkin_ws
cd ros_catkin_ws
# TODO: get rqt_rviz rviz_plugin_tutorials librviz_tutorial building w/python3
catkin config --init -DCMAKE_BUILD_TYPE=Release -DSETUPTOOLS_DEB_LAYOUT=OFF --blacklist rqt_rviz rviz_plugin_tutorials librviz_tutorial --install

# setup ROS install
rosinstall_generator desktop_full --rosdistro melodic --deps --tar > melodic-desktop-full.rosinstall
wstool init -j8 src melodic-desktop-full.rosinstall
# if packages fail to download, run the following
wstool update -j4 -t src

wstool merge -t src https://raw.githubusercontent.com/googlecartographer/cartographer_ros/master/cartographer_ros.rosinstall
wstool update -t src
rosdep update
# setup environment and install dependencies
export ROS_PYTHON_VERSION=3

#Call apt-get for each package
for pkg in "$@"
do
    echo "Installing $pkg"
    sudo apt-get -my install $pkg >> install.log
done

# and chmod it
chmod +x install_skip

# sed magic
sudo ./install_skip `rosdep check --from-paths src --ignore-src | grep python | sed -e "s/^apt\t//g" | sed -z "s/\n/ /g" | sed -e "s/python-/python3-/g"`
rosdep install --from-paths src --ignore-src -y --skip-keys="`rosdep check --from-paths src --ignore-src | grep python- | sed -e "s/^apt\t//g" | sed -z "s/\n/ /g"`"
find . -type f -exec sed -i 's/\/usr\/bin\/env[ ]*python/\/usr\/bin\/env python3/g' {} +
grep -nrw "." -e "\/usr\/bin\/python" | sed -i "s/\/usr\/bin\/\<python\>/\/usr\/bin\/python3/g"
sudo ln -sfn /usr/bin/python3.6 /usr/bin/python

catkin build