macOS Sierra (10.12) comes with Python2, but not with Python3. Please follow steps below to setup a Python3 environment:
brew search python
# install Python3
brew install python3
# create a virtual environment
python3 -m venv venv
# activate above venv
# the prompt will change after this step
source venv/bin/activate
# check the path
# it should be under ./venv
which python
# pip and pip3 can also be found ./venv,
# and they are the same
which pip
# test your env
python hello.py
hello.py
print("Hello, world!")