[Qgis-user] Cannot open QGIS3

[Miranda Voss]

I have tried installing QGIS3, but it will not open, showing: "QGIS3 cannot be opened because of a problem. Check with the developer to make sure QGIS3 works with this version of MacOSX...etc"
I am using OSX 10.13.6I have installed Python 3.6I have installed QGIS3 3.4.5 several times, I have also tried the development version (3.6). I have tried installing on a different laptop. 
I am clearly doing something quite basic wrong as there is no sign of others having difficulties on the various help forums. I would appreciate some tips.
Thank you.
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