When I've one etcd cluster separate from master, one by region (three regions), the master's write in each etcd at same time or there's one active node with other nodes at standby? The nodes switch data between them?
The main mechanism how ETCD stores key value sensitive data across Kubernetes cluster based on Raft consensus algorithm. It is a comprehensive way for distributing configuration, state and metadata information within a cluster and monitoring for any changes to the data stack.
Assuming that Master node is handling all core components inventory, it plays a role of the main contributor for managing ETCD database and has a responsibility of the leader to keep a consistent state for the other ETCD members located on worker nodes according to the distributed consensus algorithm based on a quorum model.
However, single Master node configuration does not guarantee cluster resistance to any of possible outages, therefore multi master nodes setup is more efficient way to accomplish High availability for ETCD storage as it provides consistent replica set for ETCD members distributed within a separate nodes.
In order to establish data reliability it is important to periodically backup ETCD cluster via built-in etcdctl command line tool or making snapshot for the volume where ETCD storage located.
You might be able to find more specific information about ETCD in the relevant Github project documentation.