The tri-laser shoots three weak shots, if I recall correctly? So it would be terrible against high-armor enemies. Single high-damage shots (like large caliber, non-automatic rifles or the .44 pistol) are better against armor. (This is also why Veronica's punch is good - the power fist is a single, very high strength attack.)
(As for Deathclaws & Veronica... first time I ran into Deathclaws with her, she died to in about a second, followed very quickly by me. Of course, I only have 10 DT, so....

)
What armor are you using / how much DT do you have? Also, what difficulty are you on? If you're on Hard or V.Hard, your damage is being reduced, so you have even more trouble getting through armor.
Edit: example of the tri-laser concept -
You have two weapons, one that fires 3 shots of 10 dmg at once, one that fires a 30 damage shot.
Against an unarmored enemy, they both do 30 total damage.
Against a DT 8 enemy: First gun does (10-8)x3 = 6 damage. Second gun does (30-8) = 22 damage.