Quorum devices are not required when a cluster includes more than two hosts, as the cluster survives failures of a single host without a quorum device. However, under these conditions, you cannot start the cluster without a majority of hosts in the cluster.
You can add a quorum device to a cluster that includes more than two hosts. A partition can survive as a cluster when that partition has a majority of quorum votes, including the votes of the hosts and the quorum devices. Consequently, when adding a quorum device, consider the possible host and quorum device failures when choosing whether and where to configure quorum devices.