MONs are added via a new ceph-mon unit.
Ensure that the cluster is in a healthy state. On any MON (such as the leader unit here) run a status check:
juju ssh ceph-mon/leader sudo ceph status
Add a containerised ceph-mon unit to existing machine #3:
juju add-unit --to lxd:3 ceph-mon
Recall that a cluster should always have an odd number of MONs.