RAID 5 on a new server

I’ve a complete lack of understanding of RAID. :o(
I have a new server at scaleaway which supposedly say 3 x 6TB set up as RAID 5.

A - SATA - 5.46 TB
B - SATA - 5.46 TB
C - SATA - 5.46 TB

however ,in Virtualmin I see this:
the drive status in the control panel shows as this:
/ ext4 95% (10.27 TiB) 10.82 TiB UUID=916e116b-de11-4459-b30f-516ac4d91e2d
/boot ext4 81% (782.08 MiB) 968.27 MiB UUID=d14d9051-b9c6-4131-9aed-aa6311f3426e

and at command prompt I see this:
root@ns dev]# df -H
Filesystem Size Used Avail Use% Mounted on
devtmpfs 34G 0 34G 0% /dev
tmpfs 34G 0 34G 0% /dev/shm
tmpfs 34G 9.6M 34G 1% /run
tmpfs 34G 0 34G 0% /sys/fs/cgroup
/dev/sda3 12T 2.5G 12T 1% /
/dev/sda2 1.1G 127M 821M 14% /boot
tmpfs 6.8G 0 6.8G 0% /run/user/1000

I’m confused at how the system sees it as 12TB instead of 6TB.
I have another server which also has incorrect space allocation and I would like to know if I should do something ot it should work out of the box.

thanks for any advice.
Brian

@BrianD - I think for 3 spindles set up as Raid5 (Parity) the usable space is supposed to be 12TB.

http://www.raid-calculator.com/default.aspx

might help.

Dibs

ok, thanks. I obviously know nothing about the structure, so will need to dig deeper
thanks for the link