mdadm to replace a failed drive – getting “does not appear to be an md device”

I have a 4 drive raid 5 array using mdadm. sda3 sdb3 sdc3 sdd3

sda3 failed previously.

These are all 2TB disks.

I found this post and have been following it: mdadm unmark drive as spare?

steps I performed, after starting the array in its current condition:

mdadm: array /dev/md0 started/
mdadm --detail /dev/md0 


0  Removed
1  active sync /dev/sdb3
2  active sync /dev/sdc3
3  active sync /dev/sdd3

I tried:

 mdadm --add /dev/sda3 /dev/md0

and got

  mdadm: /dev/sda3 does not appear to be an md device

do I need to –remove the /dev/sda3 before trying the add command?

I had partitioned sda3 onto this new, empty disk 0