Can't expand linear RAID on top of 2 x RAID1

I have expanded my system over time, I started with 2 x 2TB drives in
RAID1 (md2)

I then added 2 x 750GB drives, configured as RAID1 (md1)
Then created a third raid (md3) as linear with the md2 + md1

Finally, I've upgraded the 2 x 750G to 2 x 1TB drives (one at a time).

I then did a mdadm --grow to expand the RAID1 from 750G to 1TB

The problem I am having is that I can't expand the linear (md3) array to
grow the extra 250G of space.

Could anyone suggest how I might be able to get the extra 250G of space
to become available?

keep:~# cat /proc/mdstat
Personalities : [raid1] [linear]
md3 : active linear md2[0] md1[1]
      2686081193 blocks super 1.2 0k rounding
     
md1 : active (auto-read-only) raid1 sdb1[2] sdd1[3]
      976758977 blocks super 1.2 [2/2] [UU]
     
md2 : active (auto-read-only) raid1 sda1[0] sdc1[2]
      1953510841 blocks super 1.2 [2/2] [UU]
     
unused devices: <none>

keep:~# mdadm --grow /dev/md3 --size=max
mdadm: component size of /dev/md3 has been set to 0K

There is nothing logged for dmesg after the above command, no change to
/proc/mdstat.

I have tried a reboot with no change either.

Thanks for any assistance, I'd really appreciate it.

Regards,
Adam