According to Sage's FAQ:
http://www.sagemath.org/doc/faq/faq-usage.html#can-i-use-sage-with-python-3-x
"Until SciPy is ported to run with Python 3.x and Cython supports Python 3.x, Sage will continue to use Python 2.x"
Scipy has been ported to Python 3.x, and Cython already works with Python 3.x. Are there any plans to port Sage to Python 3.x?