Considering that you've already installed python3-setuptools (by running sudo apt-get install python3-setuptools) do the following
Then check the version of pip to confirm that it's installed:
More info here on how you can switch back to Python 2's pip if and when needed: https://askubuntu.com/questions/412178/how-to-install-pip-for-python-3-in-ubuntu-12-04-lts
Also, I'm not sure if python3-pyqt5 package is available for 12.04, but I guess you'll either have to follow the guide here: http://pyqt.sourceforge.net/Docs/PyQt5/installation.html#installing-from-wheels to install pyqt5 or build it by building and installing SIP and then building PyQT for python 3 by following the guide here: http://pyqt.sourceforge.net/Docs/PyQt5/installation.html#building-and-installing-from-source