the thing with three decks is (to me):
once you started playing 3, you won´t be satified with playing two anymore, and that´s why you put to much pressure onto yourself (cause you think you have to use the 3deck) and mess everything up...
the other thing is it bores you to play 2 decks, cause you need to wait that long to make the next mix....

to be honest i don´t know any bad 3 deck dj´s, give some examples