mdadm --manage /dev/md3 --add /dev/sda1

cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md0 : inactive sdd2[1] sdd1[2](S)
      31270272 blocks
      
md3 : active raid1 sda1[2] sdb1[1] sdc1[3](F)
      943730240 blocks [2/1] [_U]
      [>....................]  recovery =  4.9% (46270656/943730240) finish=217.0min speed=68900K/sec
     
unused devices:

Important Install GRUB on the new drive or you won't be able to boot from it:

grub-install /dev/sda

This is critical because you won't be able to boot off the new drive if the other fails (you'll likely just see a flashing cursor after your system posts and will wonder what the issue is).

Change /dev/sda to whatever the new drive that you added is (and I do mean sda and not sda1). 

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