Adding more control nodes adds more complexity than adding compute nodes. Generally, when additional control nodes are added, each of the control services need to be scaled and the database and message bus need to become highly available.
Triple-O has the capability to install and configure highly available scaled services for you. There is a wealth of documentation on OpenStack's Triple-O Wiki at http://docs.openstack.org/developer/tripleo-docs/. Be sure to take a look at the documentation there if you are trying to deploy something that has not been covered in this book.
As long as there are bare-metal nodes available, just a few extra parameters need to be added to the overcloud deploy command to deploy a multi-control node deployment. It is not recommended to update an existing control tier. It is recommended to plan ahead accordingly and deploy all the control nodes for the first using these parameters. Here is an example of a deployment that would have three...