@modifiedcontent
What @rcheesley mentioned is completely correct. You’ll need to download the full 3.0.2 package for the upgrade from 2.x to 3, due to some file system changes. The direct link to that package is https://github.com/mautic/mautic/releases/download/3.0.2/3.0.2.zip.
Nevertheless, I’m curious why the script isn’t working properly for you. If you try curl https://github.com/mautic/mautic/releases/download/3.0.2/3.0.2.zip --output mautic-3-update-package.zip, does that work? Then the upgrade script should also work, as it will try to download the package from that URL.
Hope we can get you up and running with Mautic 3 this way! 
Do let us know if you run into any other issues.