in ~/.nanorc you may add

set tabsize 3
set autoindent

if you want to simplify config files editing
P.S. be careful with python and tabs:
http://www.secnetix.de/olli/Python/block_indentation.hawk
http://codeghar.wordpress.com/2008/11/18/python-tabs-or-spaces/
http://www.cs.caltech.edu/courses/cs11/material/python/misc/python_style_guide.html
http://www.python.org/dev/peps/pep-0008/
http://pthree.org/2007/01/31/python-and-the-horrendous-tab-character/