The third Laval Destroyer seems like a bad idea. Just because one loses a few games to bad draws doesn't mean one should change one's deck. That third Destroyer will be useless the majority of the time, as you'll be probably be waiting for Fire and Aether quanta to build up, a Fractal, or a Quintessence. Those few times where that extra bit of damage does save you probably isn't worth giving up those games where you would've won if only you'd drawn one more card.
Also, since you're probably Fractaling your Destroyers, the third Destroyer doesn't change your damage output all that much. It's an increase from, say, 8 to 9 Destroyers.