The easiest way I've found to install pip3 (for python3.x packages) on CentOS 7 is:
$ sudo yum install python34-setuptools
$ sudo easy_install-3.4 pip
You'll need to have the EPEL repository enabled before hand, of course.
You should now be able to run commands like the following to install packages for python3.x:
$ pip3 install foo
