fdisk is the wrong tool for disks >2TB. Use parted or gdisk instead.
It appears that /dev/sdc1 and /dev/sdd1 are 2TB partitions, so that's what limits your array size. For the other disks, they have GPT so I assume they are 3TB already, but you should check.
Basically you have to stop the array, enlarge each partition to 3TB (without changing the starting offset), then start it again and follow it up with a grow:
mdadm --grow /dev/md0 --size=max
If you can't stop the array, you'll have to fail each disk2TB partition individually, repartition it and re-add it. This might go faster if you add a write-intent bitmap first.
mdadm --grow /dev/md0 --bitmap=internal
Then for each disk individually,
mdadm /dev/md0 --fail /dev/disk1 # check mdstat for [UUUU] first
mdadm /dev/md0 --remove /dev/disk1
parted /dev/disk -- mklabel gpt mkpart primary 1mib -1mib
mdadm /dev/md0 --re-add /dev/disk1
mdadm --wait /dev/md0 # must wait for sync
Once that's done you can remove the bitmap again (keeping it may harm performance).
mdadm --grow /dev/md0 --bitmap=none
mdadm --grow /dev/md0 --size=max
Finally do your resize2fs or whatever.