There are two ways to let the third one join the existing network. First is to use the pair button. Here is the instruction http://www.tp-link.com/en/article/?faqid=319
Second is to use the Utility. You can use the Management Utility to change the network name of the power line adapters. Here is an example instruction:
Only with the same Network Name, can they find and communicate with each other. To find out the Network Name, Please connect your Powerline devices to the computer. Run the Powerline Utility, go to Privacy tab, then you will see the Network Name as below:






If the Private Network Names are different, please change each device’s name to the same one, or click “Use Default (Public Network)” to make it the default name. Then, click the “Set Local Device Only” button;