With Ubuntu 18.04.2, you get python 3.6.7 pre-installed. To check it, you simply launch the terminal and run python version check command
python3 -V
or
python3 --version
If you still want to install python3.6 on your Ubuntu 18.04, please run the terminal command below
sudo apt update && upgrade
sudo apt install python3
To install pip3 on Ubuntu 18.04, run the following terminal command
sudo apt install python3-pip