What do you mean 'unused' replica shards as it should remove them when you reduce the replica count?
Also why 5 primary shards, just for sizing to say under 50GB/shard? You only have 3 nodes, so I'd think 1-2 shards would be optimal (1 for simplicity unless you have performance issues). Usually add shards to spread among nodes or for max sizing, else shards+replicas > nodes not very useful.