I installed python 3.3.1 on ubuntu lucid and successfully created a virtualenv as below
virtualenv envpy331 --python=/usr/local/bin/python3.3
I find that running
export VIRTUALENVWRAPPER_PYTHON=/usr/bin/python3
and
export VIRTUALENVWRAPPER_VIRTUALENV=/usr/bin/virtualenv-3.4
in the command line on Ubuntu forces mkvirtualenv to use python3 and virtualenv-3.4. One still has to do
mkvirtualenv --python=/usr/bin/python3 nameOfEnvironment
to create the environment. This is assuming that you have python3 in /usr/bin/python3 and virtualenv-3.4 in /usr/local/bin/virtualenv-3.4.