Quoting "Petr "Qaxi" KlĂma" qaxi@seznam.cz:
I need to add the 3rd partition, so I open documentation http://www.centos.org/docs/4/html/rhel-isa-en-4/s1-storage-rhlspec.html#S2-S... (I know the RTFM ;-)
But in chapter 5.9.9.2. they want tool caled "raidhotadd"
They might want tool called raidhotadd, but you really want tool called "mdadm". Try "man mdadm" and you'll find plethora of options to do what you need. Something like "mdadm /dev/md2 -a /dev/new-partition".