As you already have an installation of RabbitMQ all you need to do is instantiate instances on new nodes. These instructions relate to the installation on a MacBook Pro running OS X Yosemite. I assume that you have already set up RabbitMQ on your local machine and want to know how to add two more nodes and cluster them. If you are having problems setting up a cluster on a single machine then the following post might help answer some questions.
