Ubuntu 18.04.3 LTS : Disk space usage issue: looks full but can't find the problem

Question

I have a problem with disk space usage on my server. I find 97% usage in dev/md3 but when I try to find which file or folder takes up space I can't find anything...

Filesystem      Size  Used Avail Use% Mounted on
udev            7,8G     0  7,8G   0% /dev
tmpfs           1,6G  9,0M  1,6G   1% /run
/dev/md3         20G   18G  581M  97% /
tmpfs           7,9G     0  7,9G   0% /dev/shm
tmpfs           5,0M     0  5,0M   0% /run/lock
tmpfs           7,9G     0  7,9G   0% /sys/fs/cgroup
/dev/md2        487M  147M  311M  33% /boot
/dev/md4        1,8T  311G  1,4T  19% /home

4,0K    ./srv
9,0M    ./run
8,0K    ./snap
4,0K    ./mnt
1,5G    ./usr
7,3M    ./etc
26M ./sbin
311G    ./home
260M    ./var
4,0K    ./opt
116K    ./root
4,0K    ./lib64
64K ./tmp
861M    ./lib
0   ./dev
du: cannot access './proc/3718/task/3718/fd/4': No such file or directory
du: cannot access './proc/3718/task/3718/fdinfo/4': No such file or directory
du: cannot access './proc/3718/fd/3': No such file or directory
du: cannot access './proc/3718/fdinfo/3': No such file or directory
0   ./proc
16K ./lost+found
4,0K    ./media
0   ./sys
145M    ./boot
15M ./bin
314G    .

Disk /dev/sda: 1,8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: A9EF77EC-EC51-403D-B07F-1A4832C18250

Device          Start        End    Sectors    Size Type
/dev/sda1          40       2048       2009 1004,5K BIOS boot
/dev/sda2        4096    1050623    1046528    511M Linux RAID
/dev/sda3     1050624   42008575   40957952   19,5G Linux RAID
/dev/sda4    42008576 3905972223 3863963648    1,8T Linux RAID
/dev/sda5  3905972224 3907018751    1046528    511M Linux swap


Disk /dev/sdb: 1,8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 4441E7B0-2838-45FE-83DC-8B6DC3BB05B2

Device          Start        End    Sectors    Size Type
/dev/sdb1          40       2048       2009 1004,5K BIOS boot
/dev/sdb2        4096    1050623    1046528    511M Linux RAID
/dev/sdb3     1050624   42008575   40957952   19,5G Linux RAID
/dev/sdb4    42008576 3905972223 3863963648    1,8T Linux RAID
/dev/sdb5  3905972224 3907018751    1046528    511M Linux swap


Disk /dev/md2: 511 MiB, 535756800 bytes, 1046400 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/md4: 1,8 TiB, 1978349322240 bytes, 3863963520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/md3: 19,5 GiB, 20970405888 bytes, 40957824 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

/home seems to be the heaviest but it is located on a 1TO partition..

With this information I see that I have more or less 15GB which are counted as used but which do not appear anywhere.

How could I identify which files are taking up space and delete them to get free disk space on /dev/md3?