If you already have python3 installed as well virtualenvwrapper the only thing you would need to do to use python3 with the virtual environment is creating an environment using:
which python3 #Output: /usr/bin/python3
mkvirtualenv --python=/usr/bin/python3 nameOfEnvironment
Or, (at least on OSX using brew):
mkvirtualenv --python=`which python3` nameOfEnvironment
Start using the environment and you’ll see that as soon as you type python you’ll start using python3