I signed up freely at MongoDB Atlas and created cluster now I want to know how can I create database and connect to that using Robomongo?
Direct connections do not work with Replica Sets and Robo3T.
And the cluster you create on Atlas is a 3-Node replica set.
Select Connection Type: Replica Set on the first tab
To find out 3 members in new Atlas dashboard:
you will see the list of your set (primary and two secondary).
then follow @Balasubramani M's answer.