Running the following:
virtualenv -p python3 venv
gives:
Running virtualenv with interpreter /usr/bin/python3 Using base prefi
Your stack trace indicates you are using virtualenv from your 2.7 installation. This would indicate that your PATH is not set for 3.x. Start a new shell, set the PATH so that /usr/local/lib/python2.7 is not on it, and try again.