keeping python 3 up to date on a mac
I personally use Homebrew, which is a very nice package manager.
If you want to give this a try, first of all make sure you have the XCode Command Line Tools installed on your Mac.
Then install Homebrew by typing

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


in the Terminal.
Homebrew will place its "Cellar" (the set of all the packages you installed with Homebrew) in /usr/local/Cellar.

Now, about Python 3, once Homebrew is installed simply type

brew install python3


and there we go. You have Python3 installed on your Mac.

Notes:


this version will automatically take place instead of the OS X version (if it doesn't, just run brew link python3 - this will symlink python3 into /usr/local/bin/)
to update python run brew update in the Terminal (this will update Homebrew) and then brew upgrade python3 if a new version of python3 is found by the command brew update
at the end you can run brew cleanup python3 to remove every old version
with this fancy package manager, you can also take care of R