I installed Manjaro as third system and it sucsessfully detected Debian. But when I try to launch Bliss OS, I get an error 'Unknown Filesystem, load kernel first'.Usually the last distribution you install will install its grub, if asked to install grub that is. I'm not familiar with Bliss, but you could have a look at your grub configs in /etc/default/grub and check to see if the config: GRUB_DISABLE_OS_PROBER is there. If it is, configure it to false: GRUB_DISABLE_OS_PROBER=false. If the config isn't there, then write it in, and then update grub with the relevant update command from Bliss. That should enable the prober to run and if all goes well, all installations that grub can boot should be displayed.