installing pip3 (for python3) on ubuntu 16.04 LTS using a proxy

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 fixed such problem by changing Server in "Software & Updates" -> Download from: there I chose different address and 404 disappeared.

what worked for me

curl -sS https://bootstrap.pypa.io/get-pip.py >>setup.py
python3 setup.py