OK, there is contradiction. In mdadm.conf you have the default line
DEVICE partitions containers
commented out (it has # at front which makes it disabled). Later in the config you have statement to accept as devices sdb1 and sdc1 only. So it will ignore all others. I would comment out this line specifying sdb1 and sdc1 (put an # at front) and uncomment the generic DEVICE line (remove the #).
That will make it use all partitions that have mdadm superblock, not only sdb1 and sdc1. That is what you want to do, expand the array.
On the other hand, contradicting the mdadm DEVICE line that specifies sdb1 and sdc1 only, the --examine details show for both partition that they belong to a device with 3 members (Raid Devices : 3 line). Also, the examine didn't detect any superblock on sdd1. Maybe it wasn't created because the mdadm.conf doesn't accept any partitions except sdb1 and sdc1. Maybe because of another reason.
Here is what I would do:
1. Edit mdadm.conf to comment out one DEVICE line and uncomment the other, as explained above. Try the automatic assemble again:
2. The assemble will probably not work. Since the examine shows the superblock on sdb1 and sdc1 already shows 3 raid devices, try assembling it by hand with:
sudo mdadm --assemble --scan
Make sure you use the exact order of the partitions, that is very important with mdadm.
sudo mdadm --assemble /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1
3. If that doesn't work (it might not since sdd1 doesn't have a superblock), try forcing it:
See how that goes.
sudo mdadm --assemble --force /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1