Welcome to the MongoDB Community Forums,
Thanks for asking such a nice question.
I would simply say
Mongodb instance is a single copy of the software running on a single server. It has a single configuration (ideally in a configuration file) that it uses to define where it keeps its files and what port it answers on when something tries to connect to it.
However you can have multiple instances on one server. In that case, they can share the same software package but they cannot share a port, they cannot share data files, and they don’t have the same configuration (which defines those things and others).
So the unique thing is the configuration, and from there, the instance runs as a unique process.
Replica set is a group of mongod instances that host the same data set, means 3 or (any odd numbered upto 50) instance (also called members) running on either same server or different servers.
For more info read here
cluster just consists of multiple instances and in the context of MongoDB, “cluster” is the word usually used for either a replica set or a sharded cluster.
conclusion is 3
instance means 3 process running on same or different server with separate configuration files is called a replica set. And by saying
cluster we are referring to the same
Hope it clarifies your doubt.
In case of any further questions please feel free to reach out.