If you can't install OpenCV3 for Python3 on macOS via homebrew, installing numpy first will work (maybe).
$ brew install numpy --with-python3
$ brew tap homebrew/science
$ brew install opencv3 --with-contrib --with-python3 --without-python --HEAD
$ echo "/usr/local/opt/opencv3/lib/python3.6/site-packages/" \
> /usr/local/lib/python3.6/site-packages/opencv3.pth
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