NSX manager cluster needs majority to be healthy, so in a 3 node cluster only one node failure is tolerated. If you have a single node your cluster is down. It is possible to disable cluster in situations other managers cannot be recovered so you revert to a single-node manager.
Here is relevant info from NSX Manager Cluster Requirements for Single, Dual, and Multiple Sites
As of NSX-T Data Center 2.4, the NSX Manager contains the NSX Central Control Plane process. This service is critical for the operation of NSX. If there is a complete loss of NSX Managers, or if the cluster is reduced from three NSX Managers to one NSX Manager, you will not be able to make topology changes to your environment, and vMotion of machines depending on NSX will fail.