Kyngchaos installer is not finding python3. I installed python3 using HomeBrew and is not located at
How do I tell the QGIS installer to recognize that python3 is installed?
I had previously installed GDAL using HomeBrew, so tried reinstalling (or installing another version wherever it might end up) thinking it might help with the situation, but the QGIS3 installer still can't find python 3.
The installation error is a popup that comes up as soon as the installer is open and says "python3 QGIS requires Python 3.6."
For non Mac users, most seem to rely on Kyngchaos to provide an installer for QGIS and he has just released the installer for v3.
Update: answered my own questions below. Don't use HomeBrew unless you know enough to point the QGIS installer/QGIS to the right version (which I certainly don't), but use the python3 installer version linked to in the ReadMe.