Please see my answer here on how to create a raid1 array using mdadm which you can boot from (well you won't strictly speaking boot from the raid itself): Creating a partitioned raid1 array for booting a debian squeeze system
Given you have sdc to f with 1 partition covering the whole disk creating a raid5 is even simpler, example:
mdadm --create /dev/md1 --level=5 --raid-devices=4 /dev/sd[cdef]1
Or with a spare, you need 5 disks in this example, but you can create a raid5 with just 3 disks:
mdadm --create /dev/md1 --level=5 --raid-devices=4 --spare-devices=1 /dev/sd[cdefg]1
Syntax may not be exactly correct since I didn't try it out.