Wouldn't the laser rifle have an advantage against DT?
D vs DT rather than D/3 vs DT?
Or does it not work like that?
The Tri-Beam has a higher base damage and few foes have enough DT to block the shots if you're using MC cells, so the D vs D/3 difference isn't as profound as you might think. That does depend on range to target, though, since the longer the distance the higher the chance that the separate beams from the Tri-Beam will miss, due to greater spread.
It's something of a futile comparison anyway, since the two weapons aren't meant to be used in the same way. The Tri-Beam is very much an EW shotgun, and should be used accordingly, while the Laser Rifle is meant for longer ranges, to include sniping if you have the scope.