On the recent version (from Yosemite or El Capitan I believe... at least from Sierra onward), you need to run brew postinstall python3 after brew install python3 if you use homebrew.
So,
brew install python3 # this only installs python
brew postinstall python3 # this installs pip
UPDATED - Homebrew version after 1.5
According to the official Homebrew page:
On 1st March 2018 the python formula will be upgraded to Python 3.x and a python@2 formula will be added for installing Python 2.7 (although this will be keg-only so neither python nor python2 will be added to the PATH by default without a manual brew link --force). We will maintain python2, python3 and python@3 aliases.
So to install Python 3, run the following command:
brew install python3
Then, the pip is installed automatically, and you can install any package by pip install .
