When wired that way, the third speaker will only put out the difference of the two other speakers. So if you are running mono, you will hear nothing from the third speaker. If you are running a stereo source, but there are a lot of sounds that are balanced between the two main speakers, you will not hear much. If you are running your typical digital mix, you will get some echo or effects out of #3.
I know that they did the Caspian that way - but it only had two speakers and the sub. It worked fine once you figured out how to get the balance for the sub by fading from front to back. I guess the third speaker is just sort of gravy. You could swap the wires from one of the other main speakers to make #3 into one of the mains if you end up sitting near it all the time.