Thank you very much for your great tutorial. But I would like to add something from myself. We can install Ubuntu without making any additional partitions. We should only left a free space on a hard disk. Then (while the Ubuntu installation procedure) we should select the option responsible for using the larges free space for Ubuntu. Instaler will cut this free space for a three partitions, and sets Ubuntu as a default OS. Then we have to modify yaboot.conf (as boot_os wrote before) to have an access to MorphOS and MacOSX from yaboot.
To set default OS we have to change the last line before </BOOT-SCRIPT>. For example to set MacOSX as a default OS, change it to:
" "(0d 0a)" .printf bootmacosx
And don`t forget to type “hfssetmacboot <drive>:ofboot.b” in CLI on MorphOS