The requirement of minimum 3 hosts comes from the fact that Kubernetes HA cluster uses etcd for storing and syncing configuration. etcd requires minimum 3 nodes to ensure HA. In general case we need to use n+1 model when want to deploy Kubernetes HA cluster

In a single master setup, the master node manages the etcd database, API server, controller manager and scheduler, along with the worker nodes. However, if that single master node fails, all the worker node fail as well and entire cluster will be lost. In a multi-master setup, by contrast, multi-master provides high availability for a single cluster and improves network performance because all the masters behave like a unified data center. A multi-master setup protects against a wide range of failure modes, from a loss of single worker node to the failure of the master node’s etcd service. By providing redundancy, a multi-master cluster serves a highly available system for your end users.

Do not use a cluster with two master replicas. Consensus on a two-replica cluster requires both replicas running when changing persistent state. As a result, both replicas are needed and a failure of any replica turns cluster into majority failure state. A two-replica cluster is thus inferior, in terms of HA, to a single replica cluster.

Here are useful documentation: kubernetes-ha-cluster, creating-ha-cluster.

Articles: ha-cluster, ha.