sudo mdadm --remove /dev/md0

Then edit the /etc/mdadm/mdadm.conf file and comment out the RAID definition.

/etc/mdadm/mdadm.conf

#ARRAY /dev/md0 level=raid5 num-devices=3 metadata=1.2 spares=1 name=bionic:0 UUID=76c80bd0:6b1fe526:90807435:99030af9
#  devices=/dev/sdb1,/dev/sdc1,/dev/sdd1

Also, edit /etc/fstab file and comment out the line that enables auto-mount of the RAID device.

mdadm --stop /dev/md0
mdadm --zero-superblock /dev/sda1
mdadm --zero-superblock /dev/sdb1
mdadm --zero-superblock /dev/sdc1