QGIS3 won't install on mac with python3.6 already installed through Ananconda
Ran into this problem myself. Creating a symbolic link to where the installer expects to find python works. I had to find where Homebrew installed Python... (note the version in the path)
sudo ln -s /usr/local/Cellar/python/3.6.5/Frameworks/Python.framework /Library/Frameworks/Python.framework
I had python 3.7 and QGIS 3.X was looking for python 3.6.
error: "qgis requires python 3.6"
to downgrade I tried many things but finally "brew prune" and installing 3.6 directly from python.org worked for me.
I had to install python 3.6 directly downloaded from www.python.org (ignoring the already installed version by Anaconda). This worked. See remark on http://www.kyngchaos.com/blog/2018/20180315_qgis_3_must_use_python.org_python_3