You can't without rewriting the DrPython to actually run on Python 3. I don't know if there even is a stable wxPython for Python 3 yet, so it might be impossible anyway.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I have python 2.7 and 3.4 installed on my ubuntu linux pc. Drpython is selecting the older version and I would like to direct it to 3.4
You can't without rewriting the DrPython to actually run on Python 3. I don't know if there even is a stable wxPython for Python 3 yet, so it might be impossible anyway.