Sound like you are using RedHat Enterprise
You can setup software raid in the partition section of the anaconda installer.
Choose to manually partition the drive. and pick "software RAID as a filesystem type.
Check off the 3 disks that you want to use in the RAID/
Check "fill to maximum allowable size"
Now create your RAID mountpoints for the RAID devices "md0"
/ 8 GB
swap 2 GB
/var 2 GB
/usr1 4 GB
/usr2 8 GB
/usr3 8 GB
/backup 8 GB
/img1 8 GB
/img2 8 GB
/img3 8 GB
/img4 Rest of the space left
Pick the RAID level in the drop down
This should help you as well