Macの下にPython 3を取り付ける.5「python 3-3.5.0 already installed,it's just not linked」エラー

1.Macの下にHomebrewでPython 3をインストールする.5(Macが持参したPythonは2.7)brew install python3
2.インストール完了後にエラーが発生しました

Error: An unexpected error occurred during the `brew link` step
The formula built, but is not symlinked into /usr/local
Permission denied - /usr/local/Frameworks
Error: Permission denied - /usr/local/Frameworks

3.最初は権限が足りないと思っていたが、sudoで再インストールしてもだめで、再インストール(sudoを使わない)でこのようなエラーが発生した.

brew install python3
Warning: python3-3.5.0 already installed, it's just not linked

4.仕方なく、brew doctorで間違いがどこにあるかチェックしてみる(一部の重要でないヒントは省略)

**Please note that these warnings are just used to help the Homebrew maintainers**
**with debugging if you file an issue. If everything you use Homebrew for is**
**working fine: please don't worry and just ignore them. Thanks!**

Warning: The /usr/local is not writable.

You should probably change the ownership and permissions of /usr/local
back to your user account.
  sudo chown -R $(whoami) /usr/local

Warning: The /usr/local directory is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. Some versions of the
"InstantOn" component of Airfoil are known to do this.

You should probably change the ownership and permissions of /usr/local
back to your user account.
  sudo chown -R $(whoami):admin /usr/local

5.提示に従ってsudo chown -R $(whoami):admin /usr/localを入力し、その後brew link python3を入力すればよい