俺です。
3shards 9nodesなRedisはコレでOK
see: https://www.terraform.io/docs/providers/aws/r/elasticache_replication_group.html
resource "aws_elasticache_replication_group" "sample" {
..省略..
cluster_mode {
replicas_per_node_group = 2 # ←ココいじるとdestroy -> create
num_node_groups = 3 # ←ココいじるとonline resync
}
}