You ideally want to have 3 master eligible nodes in the cluster, so I would recommend making all 3 nodes master eligible. Running with a single dedicated master node makes it a single point of failure. Once you have 3 master eligible nodes, you need to set minimum_master_nodes to 2 (instead of the default 1) in order to prevent split brain scenarios.
Those are the default settings, so no need to specify tin the config. If you change it, do it on all nodes.
Even for larger clusters you can keep 3 nodes master eligible, which means that you do not need to update any settings. If you however increase the number of master eligible nodes, you must update 'minimum_master_nodes' to be (Number of master eligible nodes/2)+1 (a majority of your master eligible nodes).