This page explains two different approaches to setting up a highly available Kubernetes cluster using kubeadm:

With stacked masters. This approach requires less infrastructure. etcd members and control plane nodes are co-located.
With an external etcd cluster. This approach requires more infrastructure. The control plane nodes and etcd members are separated.

Your clusters must run Kubernetes version 1.11 or later. You should also be aware that setting up HA clusters with kubeadm is still experimental. You might encounter issues with upgrading your clusters, for example. We encourage you to try either approach, and provide feedback.