This to setup you own environment.

1. On server A, B, and C, make sure you install version  4 from
https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/
made for apt-get.

2. If you have problems,
apt-get remove mongodb-org-mongos mongodb-org-server mongodb-org-shell mongodb-org-tools
apt-get purge mongodb-org-mongos mongodb-org-server mongodb-org-shell mongodb-org-tools
 
sudo apt-get install -y  mongodb-org

3. Server A, B, C
to /etc/mongodb.conf add the two lines
replication:
  replSetName: r1

4. To all servers,
sudo service mongod stop
sudo service mongod start

5. Make sure they not part of anby repicat set, on each server. 
mongo --eval "rs.status()"

6. First server start mongo
rs.initiate()
rs.status()    # You should see your first server is the only member of the set.
rs.add('<server b>')
rs.status()  # 2 memebers should not exist. 
rs.add('<server c>')
rs.status()  # 3 memebers should not exist.