That's a COMPLETELY TERRIBLE idea, and you should never do that. You're likely to break dependencies and requirements for various software that expect to find the default Python in the default location.
Install your choice of Python 3 ( MacPorts, Python.org, whatever ). Update your local $PATH variables to use that Python / PythonPath.
If you don't trust my opinion, here's what official Python.org docs say:
http://docs.python.org/2/using/mac.html
The Apple-provided build of Python is installed in /System/Library/Frameworks/Python.framework and /usr/bin/python, respectively. You should never modify or delete these, as they are Apple-controlled and are used by Apple- or third-party software. Remember that if you choose to install a newer Python version from python.org, you will have two different but functional Python installations on your computer, so it will be important that your paths and usages are consistent with what you want to do.