Backing up a MongoDB cluster with mongodump? Can anyone indicate the right way to backup a MongoDB database with mongodump?

I have tried:

mongodump --host "cluster0-shard-00-00-2djno.mongodb.net:27017,cluster0-shard-00-01-2djno.mongodb.net:27017,cluster0-shard-00-02-2djno.mongodb.net:27017" \
  --authenticationDatabase admin --ssl --username admin \
  --password mypassword -o /tmp


When I do that I get:

Failed: error connecting to db server: no reachable servers


Verifying connection to an individual node with both the ping and telnet commands such that these nodes are valid.

The mongodump version is 3.2.13.
Upgrading to MongoDB 3.4 was necessary, since the cluster string did not appear to work with 3.2.

Also, based on the blog post for 'importing data', a small tweak to the host parameter seemed necessary, in the form of adding the 'Cluster0-shard-0/' prefix :

mongodump --host "Cluster0-shard-0/cluster0-shard-00-00-2djno.mongodb.net:27017,cluster0-shard-00-01-2djno.mongodb.net:27017,cluster0-shard-00-02-2djno.mongodb.net:27017" \
  --authenticationDatabase admin --ssl --username admin \
  --password mypassword --oplog -o /tmp


Edit: updated the response based on @jjussi's feedback. Also, we aren't trying to avoid using the mongo cloud backup, which is more convenient, it is that we just feel better having our own local copy, for various reasons.