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
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
Watch me Install python3 and pip3 on Ubuntu 18.04