Resize RAID partition with GPT partition layout, without LVM

As you are using EXT4, it should be possible to shrink the /home/ partition. Let do an example shrinking it to about 2 TB:

1. unmount your filesystem with umount /dev/md3
2. check your filesystem with fsck /dev/md3
3. resize the filesystem with resize2fs /dev/md3 1800G
4. resize the RAID device with mdadm --grow /dev/md3 --size=1900G Please note that I left the array bigger than the underlying filesystem, by a great margin. This is because the last thing you want is to shrink too much your array, at a point where the underlying filesystem can't fit in it. This is a very bad scenario, with almost 100% guaranteed data loss.
5. recheck your filesystem with fsck /dev/md3
6. remount your filesystem and try to read/write to/from it.

Here you can find some other information.

Anyway, if your /dev/md3 device is almost empty, maybe destroying/recreating the array/partitions can both be easier and faster.