sudo apt-get install python3-pip code example

Example 1: install pip ubuntu

sudo apt install python3-pip

Example 2: sudo apt-get install python3.5

UPDATE

Thanks to Don Kirkby who highlight that there is a new recommended PPA.
The updated one is ppa:deadsnakes/ppa, so the complete command is the following:

sudo add-apt-repository ppa:deadsnakes/ppa
If you are using Ubuntu, you can install it adding a PPA; so you can proceed in this way:

sudo add-apt-repository ppa:fkrull/deadsnakes
sudo apt-get update
sudo apt-get install python3.5
If this procedure doesn't work, is possible that your Ubuntu version not supported it. So i think that you can install in this way:

sudo apt-get install libssl-dev openssl
wget https://www.python.org/ftp/python/3.5.0/Python-3.5.0.tgz
tar xzvf Python-3.5.0.tgz
cd Python-3.5.0
./configure
make
sudo make install

Example 3: sudo pip3 install

[server]$ pip3 --version
pip 18.1 from /home/username/opt/python-3.6.2/lib/python3.6/site-packages/pip (python 3.6)

Example 4: install pip ubuntu

pip3 --version