python3, and their associated libraries as different packages (rather than one being an upgraded version of the other), since there are fairly few libraries which have a common codebase executable in both python 2 and 3.
Thus, you want
python3-matplotlib, etc. Unfortunately,
python3-matplotlib is not available in 12.04 (it is available from 13.04). You might be able to find a PPA containing it, but I personally don't know one to recommend.
(Other libraries you might want to consider for a scientific python setup might include
pandas, and the various
Edit: Learning python 3 rather than 2 is a sensible goal, but if you're restricted to 12.04 you're likely to find support lacking in a number of libraries. That being the case, the best course is probably to use python 2 while ensuring you write forward-compatible code - see the official porting guide for suggestions.