I would have to agree with Dustin Zahn 100% - that using the 3rd deck to cue your tracks faster is definately what should be done if the DJ is only playing on 2 decks. As he also mentioned, there are no rules on what exacltly you need to do to use three decks.

In my opinion 3 or 2 decks, whatever, if you mix the tunes well in your own style then keep doing it, you do it the way you feel comfortable doing it.