What causes AppleScript error 1721 when trying to execute Python3 file?
Thank you  user3439894 on the basis of your answers I was able to figure out the problem.

That's the correct script for AppleScript:

tell application "Terminal"
    do shell script "/usr/local/bin/python3 /Users/UserName/Desktop/PyHello.py $@"
end tell


This script is for Automator to run AppleScript:

on run {input, parameters}
    tell application "Terminal"
        do shell script "/usr/local/bin/python3 /Users/UserName/Desktop/PyHello.py $@"
    end tell
    return input
end run


Also following shebang is needed in initial Python script: 

#!/usr/bin/env python3