I have both python2.7 and python3.2 installed in Ubuntu 12.04.
The symbolic link python links to python2.7
Execute the pip binary directly.
First locate the version of PIP you want.
jon-mint python3.3 # whereis ip
ip: /bin/ip /sbin/ip /usr/share/man/man8/ip.8.gz /usr/share/man/man7/ip.7.gz
Then execute.
jon-mint python3.3 # pip3.3 install pexpect
Downloading/unpacking pexpect
Downloading pexpect-3.2.tar.gz (131kB): 131kB downloaded
Running setup.py (path:/tmp/pip_build_root/pexpect/setup.py) egg_info for package pexpect
Installing collected packages: pexpect
Running setup.py install for pexpect
Successfully installed pexpect
Cleaning up...