You can set up a dedicated cluster for monitoring purposes only and monitor each of those 3 cluster in one view as it's a part of X-Pack monitoring feature under a paid subscription including ECE.
If there is no way to split the monitoring, does it mean I need to setup a monitoring cluster to each production elasticsearch cluster?
This will not be necessary as you can create a dedicated cluster for monitoring and send production cluster metrics to that monitoring cluster.
I also believe Marvel can monitor each cluster. But in Kibana-> Monitoring->Overview, I don't see any option to monitor only one cluster. The metric like QPS shows the total QPS of 3 clusters.
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