If the workflow is installed, it should show in your workflows list in the Alfred’s preferences. Does it show there? If not, do you have the Alfre Powerpack?
Next up, what happens if you install this workflow, and type pythontest abc into Alfred?
The script it runs is this:
#!/usr/bin/env python3
import sys
strArg = ' '.join(sys.argv[1:])
sys.stdout.write('START>>>')
sys.stdout.write('You entered "' + strArg + '"')
sys.stdout.write('<<<END')
If everything is in place, then you should get this displayed on the screen.
If it doesn’t work, try some Alfred workflow debugging:
https://www.alfredapp.com/help/workflows/advanced/debugger/
I have added a debug block after the script in that test workflow for convenience.
If that isn’t giving anything, my best guess would be an issue with thePython 3 installation.
If you type ls /usr/bin/py*3 in at a terminal, do you get /usr/bin/python3 as the result?