It looks like a device is completely missing?
blkid > blkid.txt
Then for each md device UUID pair (the md UUID will show up twice in
blkid for each of the three arrays):
mdadm -E <dev1> <dev2> >> mdXraidstats.txt
Where X=the md device from mdstat. That way you get the superblock
from each device making up each array into an array specific text
file. They can go all in one file, but it's easier to keep things
separate.
journalctl -b -l -o short-monotonic > journal.txt
/etc/mdadm/mdadm.conf
Post those files somewhere and the URL here.
The journal should say why it wasn't assembled. But the main thing is
you should get copies of all device superblocks before you do anything
else, especially reading around the internet where often people get
the idea that they need to use -C to recreate an array which
obliterates the old metadata and almost invariably kills their data.
Chris Murphy