At the time the cluster bootstrapped, it did not know the ID of master 3 so it uses a placeholder that will be replaced with master 3's ID later. This is quite a low-level implementation detail that almost certainly isn't relevant to your problem
You did not set cluster.initial_master_nodes to the same value on all three nodes, or you have changed it after the first time you started these nodes.
Not obviously, but if you changed cluster.initial_master_nodes then that was wrong.