# /dev/md0 is (temporary) name
# level is your raid level (raid5 here)
# raid-devices is the amount of disks/partitions followed by the partition names
mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1
# make ext4 file system from your raid partition
mkfs -t ext4 /dev/md0