I am trying to install QGIS on a MacBook, using MacOS High Sierra Version 10.13.6. For some reason, having installed Python 3.6, my Mac refuses to use the installer saying that QGIS is not a trusted developer. Is there a solution to this?
You may try alternative MacOS QGIS installer by NextGIS. At this moment 2.18.25 QGIS version only. This installer signed and indicated as trusted.
Disclosure: I'm developer at NextGIS.
Go to the system preferences and click "security".
Under the general tab you will see a warning towards the bottom of the screen saying something like "[name of program] was blocked from opening". Next to this warning there will be an Open Anyway button.
If you click this the installer will open.