Replacing failed disk in the software raid1

The disk /dev/sda is failing:
cat /proc/mdstat
Remove it from the raid:
cat /proc/mdstat
mdadm --manage /dev/md127 --fail /dev/sda1
mdadm --manage /dev/md126 --fail /dev/sda3
mdadm --manage /dev/md125 --fail /dev/sda2

after replacing the bad disk we should add the new one in to the raid:

dmsetup remove_all
sfdisk -d /dev/sdb | sfdisk /dev/sda
cat /proc/mdstat
mdadm --manage /dev/md127 --add /dev/sda1
mdadm --manage /dev/md126 --add /dev/sda3
mdadm --manage /dev/md125 --add /dev/sda2

Monitor the sync process:
watch cat /proc/mdstat

Every 2.0s: cat /proc/mdstat                                                                                                                                                                             Tue Jun 26 10:33:19 2018

Personalities : [raid1]
md125 : active raid1 sda2[2] sdb2[0]
      2099136 blocks super 1.0 [2/1] [U_]
        resync=DELAYED
      bitmap: 1/1 pages [4KB], 65536KB chunk

md126 : active raid1 sda3[2] sdb3[0]
      477894656 blocks super 1.2 [2/1] [U_]
      [===>.................]  recovery = 19.9% (95154432/477894656) finish=129.8min speed=49130K/sec
      bitmap: 4/4 pages [16KB], 65536KB chunk

md127 : active raid1 sda1[2] sdb1[0]
      8256512 blocks super 1.2 [2/2] [UU]

unused devices: