OS is Ubuntu 18.04.1 LTS.
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.1 LTS"
Default python's version is 2.x.
$ python -V
Python 2.7.15rc1
$ sudo apt-get install python3-venv
# python3 -m venv [new-venv-name]
$ python3 -m venv YOUR-VENV
# source [new-venv-name]/bin/activate
# or
# . [new-venv-name]/bin/activate
$ source YOUR-VENV/bin/activate
Confirm the version of Python.
(YOUR-VENV) $ python -V
Python 3.6.6
(YOUR-VENV) $ deactivate
Confirm the version of Python.
$ python -V
Python 2.7.15rc1