Can we, say for example, a infrastructure which runs mongodb on 3 docker containers in an EC2 instance be/migrated to a new kubernetes cluster which we intend to deploy? ( i mean the container as it is to be orchestrated under the new kubernetes cluster)
What should be the idle compute/mem/HDD to be considered for a 3 master nodes as they do not run any applications on them. and why is 3 master nodes idle over 2 master nodes?
The official documentation describes Stateful-Sets as
StatefulSets are intended to be used with stateful applications and distributed systems.
so StatefulSets is one of the best use cases for this is to orchestrate data-store services such as MongoDB and good to know RAM size is the most important factor for instance sizing.
Regarding minimum of three master nodes, you can check the answer here.