To make the path permanently available to Mathematica (frontend) and all other applications, first find out the location of your desired python3 executable (e.g. from which python3 in the Terminal), and then add this to the path variable that is set for all applications when the user logs in. Let's say the executable is in /usr/local/bin as @b3m2a1 mentioned in the comment to your question (it may be somewhere else!). Then do the following (in Terminal):

sudo launchctl config user path /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

The last four locations are the default entries, and I prepended the new location to that lies (separated by :). To make this take effect, you have to restart the computer.

To make the path permanently available to Mathematica (frontend) and all other applications, first find out the location of your desired python3 executable (e.g. from which python3 in the Terminal), and then add this to the path variable that is set for all applications when the user logs in. Let's say the executable is in /usr/local/bin as @b3m2a1 mentioned in the comment to your question (it may be somewhere else!). Then do the following (in Terminal):

sudo launchctl config user path /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

The last four locations are the default entries, and I prepended the new location to that list (separated by :). To make this take effect, you have to restart the computer.