Replica model in Cassandra vs Mongo? Say in Mongo we have one primary and two replica node(simple replica no sharding). Read and write request will be handled only primary node 
and replica will just backup the data in case of failover.

My understanding/question about replication(not sharding) in Cassandra

Can we have only one node(say Node_1) serving read and write requests while other two simply working as replica nodes(these nodes just takes the replica request from Node_1 , no real time request from user ) like in Mongo considering replication factor as 3 ?

Per my understanding this should not be possible in cassandra, as in cassandra , replica nodes are nodes are equal peers , so it is mandatory that any of three nodes has to serve write and read request.

Is my understanding correct ?
Yes, your understanding is correct - this allows evenly distribute load between nodes &amp; to handle more traffic. 

Why would you need replicas if they just idle? If you're afraid of inconsistent data, then just set correct consistency levels to get strong consistency...