GDAL scripts not found in QGIS 3 on OSX

I installed QGIS 3, and tried to execute tools like "sieve" "merge" and "polygonize". All of them gave me the error message:

GDAL command output: /bin/sh: (scriptname).py: command not found

I found the scripts in both /Username/miniconda3/bin and /Username/miniconda3/lib/python3.6/site-packages/GDAL-2.2.2-py3.6-macosx-10.9-x86_64.egg-info/scripts.

Checking env in Terminal shows that miniconda3/bin is in the path:

PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin://Users/Username/miniconda3/bin

What could be causing the failure of executing those scripts?