It seems python3-pip is not installed correctly on my system so I did

sudo apt install --reinstall python3-pip

Now I can install programs with pip3.

edit: Manual installation and use of setuptools is not the standard process anymore.

If you're running Python 2.7.9+ or Python 3.4+

Congrats, you should already have pip installed. If you do not, read onward.

If you're running a Unix-like System

You can usually install the package for pip through your package manager if your version of Python is older than 2.7.9 or 3.4, or if your system did not include it for whatever reason.

Instructions for some of the more common distros follow.

Installing on Debian (Wheezy and newer) and Ubuntu (Trusty Tahr and newer) for Python 2.x

Run the following command from a terminal:

sudo apt-get install python-pip 

Installing on Debian (Wheezy and newer) and Ubuntu (Trusty Tahr and newer) for Python 3.x

Run the following command from a terminal:

sudo apt-get install python3-pip

On a fresh Debian/Ubuntu install, the package may not be found until you do:

sudo apt-get update

Installing pip on CentOS 7 for Python 2.x

On CentOS 7, you have to install setup tools first, and then use that to install pip, as there is no direct package for it.

sudo yum install python-setuptools
sudo easy_install pip

Installing pip on CentOS 7 for Python 3.x

Assuming you installed Python 3.4 from EPEL, you can install Python 3's setup tools and use it to install pip.

# First command requires you to have enabled EPEL for CentOS7
sudo yum install python34-setuptools
sudo easy_install pip

If your Unix/Linux distro doesn't have it in package repos

Install using the manual way detailed below.

The manual way

If you want to do it the manual way, the now-recommended method is to install using the get-pip.py script from pip's installation instructions.

Install pip

To install pip, securely download get-pip.py

Then run the following (which may require administrator access):

python get-pip.py 

If setuptools is not already installed, get-pip.py will install setuptools for you.

Try this command

sudo apt-get install python3-pip

Note: If you have a fresh install, you need to do this first before trying the above command.

sudo apt-get update

2018 Update: This is still attracting upvotes which worries me.

12.04 has been unsupported for about a year now. The best way you can install pip3 is by upgrading to a newer, supported version of Ubuntu. You have 3 LTS versions to pick from.

While on more modern versions of Ubuntu you could just sudo apt-get install python3-pip (and then use pip3), a Python 3 copy of pip was never packaged for 12.04.

Therefore you need to follow the more old fashioned install route with easy_install:

sudo apt-get install python3-setuptools
sudo easy_install3 pip

Now, there is every chance that this will clash with Python 2's pip and override /usr/bin/pip, because it will install a python3 based /usr/local/bin/pip which is also in Ubuntu 12.04's $PATH.

It shouldn't overwrite it so as long as you know that, it might be acceptable. However it might be best to start investigating the happy world of virtualenv as this answer suggests.

Alternatively you could rename the easy-installed python3 version of pip:

sudo mv /usr/local/bin/pip /usr/local/bin/pip-3

Then you can confirm your existing pip is still python2.7 based:

pip --version

The first procedure you followed is correct

sudo apt-get -y install python3-pip

But before installing try to update using command

sudo apt-get update

If first did not work then you can also do this using curl

curl "https://bootstrap.pypa.io/get-pip.py" -o "get-pip.py"
python3 get-pip.py --user

Then to verify installation try

pip3 --help 

For checking version :

pip3 --version 

I believe that you can install it on Ubuntu with

Copysudo apt-get install python-pip

or

Copysudo apt-get install python3-pip

for Python 3. Sure, it's an older version but its functionality is there.