In case someone else stumbles across this thread, we are running our ArcGIS Pro, Python 3.x scripts on a Windows Server 2008 R2 machine (same machine that runs our ArcGIS Desktop/Python 2.x scripts) and it appears that you need to do a few things to be successful with running ArcGIS Pro/Python 3.x scripts:
1) Install latest version of ArcGIS Pro on the server where you're running the scripts. We're currently running ArcGIS Pro 2.3.3 at the time of this writing.
2) "Allow License Borrowing" on your ArcGIS License Server Administrator
3) On the server where you're running the scripts, log in as the user who will run the scripts, open up ArcGIS Pro, logged in as the user who will be calling this scheduled task, and go to backstage>>Licensing...and check the "Authorize ArcGIS Pro to work offline" option. Otherwise, you have to have ArcGIS Pro set to sign in as a user automatically that is the same name as a named user in ArcGIS Online (we don't have Portal).
One thing to note, that makes your scripts not work (and could make you think something else is going on), is if your ArcGIS Pro Python Package Manager is pointing to the default environment and/or your Python environment doesn't have particular libraries installed that you may be referencing in your script. So make sure you've got all those libraries installed on whatever environment you've chose in ArcGIS Pro!