If you set minimum_master_nodes to 2with 3 available master eligible nodes, only one master can be elected as 2 need to agree for that to happen. If a single node is partitioned off it will not be able to reach majority and it can therefore not elect itself master, meaning that will always be at most one (there may be none for periods while election takes place) elected master node in the cluster. There is therefore no risk of a split-brain scenario.