As you know, the Remote Extender Kit includes a 1 Eye Emitter Cable which is used in situations where you have decided to place your Receiver where it has no line of sight to control your components.
An example would be if you decided to place the receiver behind your satellite decoder where it is unable to see the front of your decoder.
You would then need to use the 1 Eye Emitter Cable and place it on the front of your decoder.
The 3 Eye Emitter Cable is used in situations as explained above, except you now have 3 Eyes which enable you to control 3 separate components.
Please keep in mind that in most situations the Emitter Cables mentioned above would NOT need to be used, unless you decide to place the Receiver where it has absolutely no sight of your components that you would like to control.