I Installed a fresh copy of Mautic 3.0.0 on a new domain. Installation went good. No errors.
After that I setup the same cron jobs as my old (2.15.2) Mautic. However, I changed the paths in the the cron jobs from /app to /bin.
But it looks like no cron job is working. Mautic 3.0.0 is completely dead and does nothing at all.