brew install python3
此时，安装完成后会出现一点问题：
liyang@liyang ~ brew install python3
Updating Homebrew…
==> Auto-updated Homebrew!
Updated 1 tap (homebrew/php).
==> Updated Formulae
homebrew/php/adminer homebrew/php/drupalconsole
==> Installing dependencies for python3: gdbm
==> Installing python3 dependency: gdbm
==> Downloading https://mirrors.tuna.tsinghua.edu.cn/homebrew-bottles/bottles/gd
################################################################## 100.0%
==> Pouring gdbm-1.14.1_1.high_sierra.bottle.tar.gz
�� /usr/local/Cellar/gdbm/1.14.1_1: 20 files, 555.7KB
==> Installing python3
==> Downloading https://mirrors.tuna.tsinghua.edu.cn/homebrew-bottles/bottles/py
################################################################## 100.0%
==> Pouring python3-3.6.4_2.high_sierra.bottle.tar.gz
Error: An unexpected error occurred during the brew link step
The formula built, but is not symlinked into /usr/local
Permission denied @ dir_s_mkdir - /usr/local/Frameworks
Error: Permission denied @ dir_s_mkdir - /usr/local/Frameworks
✘ liyang@liyang ~ ls /usr/local/Frameworks
ls: /usr/local/Frameworks: No such file or directory
此时会发现两个错误，第一个是权限问题，第二个是/usr/local/Frameworks目录不存在。
首先，我们先解决目录不存在的问题
sudo mkdir /usr/local/Frameworks
创建目录以后，我们开始解决权限问题，通过Google，发现大多数处理方法都是
sudo chown $(whoami):admin /usr/local/
虽然能解决问题，但并不是一个好的idea~,这里非常不建议使用，那么解决方案入下：
sudo chown -R
(brew –prefix)/*
此时，上面两个问题就已经安装完成，只需要最后一步link一下即可。
brew link python3
Linking /usr/local/Cellar/python3/3.6.4_2… 1 symlinks created
此时python3就已经安装完成（/usr/local/Cellar/），以后可以使用brew来更新了~
参考链接
https://github.com/Homebrew/homebrew-core/issues/19286
https://stackoverflow.com/questions/16432071/how-to-fix-homebrew-permissions